10-Q

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C.  20549

FORM 10-Q

(Mark One)

x QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

For the quarterly period ended June 30, 2018

or

¨ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

Commission File Number: 001-3280

Public Service Company of Colorado

(Exact name of registrant as specified in its charter)

Colorado	84-0296600
(State or other jurisdiction of incorporation or organization)	(I.R.S. Employer Identification No.)
1800 Larimer, Suite 1100
Denver, Colorado	80202
(Address of principal executive offices)	(Zip Code)

(303) 571-7511

(Registrant’s telephone number, including area code)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. x Yes ¨ No

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 and Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files). x Yes ¨ No

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company or an emerging growth company.  See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

Large accelerated filer ¨	Accelerated filer ¨
Non-accelerated filer x	Smaller reporting company ¨
(Do not check if smaller reporting company)	Emerging growth company ¨

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ¨

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). ¨ Yes x No

Indicate the number of shares outstanding of each of the issuer’s classes of common stock, as of the latest practicable date.

Class	July 27, 2018
Common Stock, $0.01 par value	100 shares

Public Service Company of Colorado meets the conditions set forth in General Instruction H (1) (a) and (b) of Form 10-Q and is therefore filing this Form 10-Q with the reduced disclosure format specified in General Instruction H (2) to such Form 10-Q.

TABLE OF CONTENTS

PART I — FINANCIAL INFORMATION
Item l —	Financial Statements (Unaudited)	3
Item 2 —	Management’s Discussion and Analysis of Financial Condition and Results of Operations	26
Item 4 —	Controls and Procedures	31
PART II — OTHER INFORMATION
Item 1 —	Legal Proceedings	32
Item 1A —	Risk Factors	32
Item 6 —	Exhibits	32
SIGNATURES		33
Certifications Pursuant to Section 302		1
Certifications Pursuant to Section 906		1
Statement Pursuant to Private Litigation		1

This Form 10-Q is filed by Public Service Company of Colorado, a Colorado corporation (PSCo). PSCo is a wholly owned subsidiary of Xcel Energy Inc. Xcel Energy Inc. wholly owns the following subsidiaries: Northern States Power Company, a Minnesota corporation (NSP-Minnesota); Northern States Power Company, a Wisconsin corporation (NSP-Wisconsin); PSCo; and Southwestern Public Service Company, a New Mexico corporation (SPS). NSP-Minnesota, NSP-Wisconsin, PSCo and SPS are also referred to collectively as utility subsidiaries. Additional information on Xcel Energy Inc. and its subsidiaries (collectively, Xcel Energy) is available on various filings with the Securities and Exchange Commission (SEC).

PART I — FINANCIAL INFORMATION

Item 1 — FINANCIAL STATEMENTS

PUBLIC SERVICE CO. OF COLORADO AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED)

(amounts in thousands)

	Three Months Ended June 30						Six Months Ended June 30
	2018			2017			2018			2017
Operating revenues
Electric	$	716,195		$	729,920		$	1,414,469		$	1,441,308
Natural gas	186,661			192,777			550,647			548,913
Steam and other	9,010			8,219			20,048			21,229
Total operating revenues	911,866			930,916			1,985,164			2,011,450
Operating expenses
Electric fuel and purchased power	271,891			279,522			553,061			568,349
Cost of natural gas sold and transported	58,559			70,258			249,824			266,660
Cost of sales — steam and other	3,664			3,507			7,540			7,893
Operating and maintenance expenses	188,991			187,394			372,066			372,482
Demand side management expenses	33,202			29,928			65,954			58,032
Depreciation and amortization	116,553			117,513			238,160			232,507
Taxes (other than income taxes)	49,743			49,470			102,400			99,268
Total operating expenses	722,603			737,592			1,589,005			1,605,191
Operating income	189,263			193,324			396,159			406,259
Other income, net	799			1,319			1,030			4,523
Allowance for funds used during construction — equity	13,555			6,341			24,499			10,949
Interest charges and financing costs
Interest charges — includes other financing costs of $1,601 and $1,543, $3,173, and $3,064 respectively	51,221			46,424			101,142			92,306
Allowance for funds used during construction — debt	(5,205		)	(2,438		)	(9,786		)	(4,344		)
Total interest charges and financing costs	46,016			43,986			91,356			87,962
Income before income taxes	157,601			156,998			330,332			333,769
Income taxes	35,305			56,411			74,314			121,636
Net income	$	122,296		$	100,587		$	256,018		$	212,133

See Notes to Consolidated Financial Statements

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PUBLIC SERVICE CO. OF COLORADO AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED)

(amounts in thousands)

	Three Months Ended June 30				Six Months Ended June 30
	2018		2017		2018		2017
Net income	$	122,296	$	100,587	$	256,018	$	212,133
Other comprehensive income (loss)
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $0, $1, $0, and $2, respectively	2		1		4		2
Derivative instruments:
Reclassification of losses to net income, net of tax of $99, $153, $197 and $305, respectively	303		250		603		496
Other comprehensive income	305		251		607		498
Comprehensive income	$	122,601	$	100,838	$	256,625	$	212,631

See Notes to Consolidated Financial Statements

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PUBLIC SERVICE CO. OF COLORADO AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)

(amounts in thousands)

	Six Months Ended June 30
	2018			2017
Operating activities
Net income	$	256,018		$	212,133
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	240,577			234,143
Demand side management program amortization	—			672
Deferred income taxes	25,579			126,252
Amortization of investment tax credits	(1,399		)	(1,401		)
Allowance for equity funds used during construction	(24,499		)	(10,949		)
Net realized and unrealized hedging and derivative transactions	(1,917		)	1,951
Other	1			—
Changes in operating assets and liabilities:
Accounts receivable	21,991			24,042
Accrued unbilled revenues	66,497			81,649
Inventories	44,769			38,452
Prepayments and other	1,522			(4,837		)
Accounts payable	(22,097		)	(51,894		)
Net regulatory assets and liabilities	30,405			(2,499		)
Other current liabilities	(119,132		)	(67,418		)
Pension and other employee benefit obligations	(27,457		)	(16,543		)
Change in other noncurrent assets	3,880			(717		)
Change in other noncurrent liabilities	(14,546		)	(228		)
Net cash provided by operating activities	480,192			562,808
Investing activities
Utility capital/construction expenditures	(825,463		)	(609,369		)
Allowance for equity funds used during construction	24,499			10,949
Investments in utility money pool arrangement	(198,000		)	(777,000		)
Repayments from utility money pool arrangement	56,000			625,000
Other, net	—			(657		)
Net cash used in investing activities	(942,964		)	(751,077		)
Financing activities
Repayments of short-term borrowings, net	—			(129,000		)
Borrowings under utility money pool arrangement	526,000			40,000
Repayments under utility money pool arrangement	(526,000		)	(40,000		)
Proceeds from issuance of long-term debt	692,697			394,611
Capital contributions from parent	216,508			82,475
Dividends paid to parent	(171,546		)	(161,312		)
Other	(118		)	(110		)
Net cash provided by financing activities	737,541			186,664
Net change in cash and cash equivalents	274,769			(1,605		)
Cash and cash equivalents at beginning of period	7,513			5,926
Cash and cash equivalents at end of period	$	282,282		$	4,321
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	$	(88,912	)	$	(84,452	)
Cash paid for income taxes, net	(96,448		)	(12,195		)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$	131,823		$	103,774

See Notes to Consolidated Financial Statements

5

PUBLIC SERVICE CO. OF COLORADO AND SUBSIDIARIES

CONSOLIDATED BALANCE SHEETS (UNAUDITED)

(amounts in thousands, except share and per share data)

	June 30, 2018			Dec. 31, 2017
Assets
Current assets
Cash and cash equivalents	$	282,282		$	7,513
Accounts receivable, net	272,293			294,403
Accounts receivable from affiliates	38,650			14,719
Investments in utility money pool arrangement	162,000			20,000
Accrued unbilled revenues	229,304			295,801
Inventories	169,720			214,489
Regulatory assets	76,419			77,337
Derivative instruments	5,340			3,197
Prepayments and other	39,237			35,720
Total current assets	1,275,245			963,179
Property, plant and equipment, net	14,541,078			14,025,751
Other assets
Regulatory assets	980,811			950,258
Derivative instruments	2,716			1,009
Other	23,259			27,429
Total other assets	1,006,786			978,696
Total assets	$	16,823,109		$	15,967,626
Liabilities and Equity
Current liabilities
Current portion of long-term debt	$	705,869		$	305,577
Accounts payable	433,822			492,829
Accounts payable to affiliates	38,395			58,749
Regulatory liabilities	95,053			66,126
Taxes accrued	108,812			222,517
Accrued interest	49,803			48,552
Dividends payable to parent	100,338			76,195
Derivative instruments	5,428			7,348
Other	92,583			92,333
Total current liabilities	1,630,103			1,370,226
Deferred credits and other liabilities
Deferred income taxes	1,678,207			1,644,476
Deferred investment tax credits	26,459			27,858
Regulatory liabilities	1,926,944			1,933,488
Asset retirement obligations	355,028			347,769
Derivative instruments	2,471			3,468
Customer advances	170,659			162,614
Pension and employee benefit obligations	260,012			287,783
Other	49,454			58,923
Total deferred credits and other liabilities	4,469,234			4,466,379
Commitments and contingencies
Capitalization
Long-term debt	4,594,193			4,302,698
Common stock — 100 shares authorized at $0.01 par value; 100 shares outstanding at June 30, 2018 and Dec. 31, 2017, respectively	—			—
Additional paid in capital	4,273,146			4,032,826
Retained earnings	1,882,558			1,822,229
Accumulated other comprehensive loss	(26,125		)	(26,732		)
Total common stockholder’s equity	6,129,579			5,828,323
Total liabilities and equity	$	16,823,109		$	15,967,626

See Notes to Consolidated Financial Statements

6

PUBLIC SERVICE CO. OF COLORADO AND SUBSIDIARIES

Notes to Consolidated Financial Statements (UNAUDITED)

In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary to present fairly, in accordance with accounting principles generally accepted in the United States of America (GAAP), the financial position of PSCo and its subsidiaries as of June 30, 2018 and Dec. 31, 2017; the results of its operations, including the components of net income and comprehensive income, for the three and six months ended June 30, 2018 and 2017; and its cash flows for the six months endedJune 30, 2018 and 2017. All adjustments are of a normal, recurring nature, except as otherwise disclosed. Management has also evaluated the impact of events occurring after June 30, 2018 up to the date of issuance of these consolidated financial statements. These statements contain all necessary adjustments and disclosures resulting from that evaluation. The Dec. 31, 2017 balance sheet information has been derived from the audited 2017 consolidated financial statements included in the PSCo Annual Report on Form 10-K for the year ended Dec. 31, 2017. These notes to the consolidated financial statements have been prepared pursuant to the rules and regulations of the SEC for Quarterly Reports on Form 10-Q. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP on an annual basis have been condensed or omitted pursuant to such rules and regulations. For further information, refer to the consolidated financial statements and notes thereto, included in the PSCo Annual Report on Form 10-K for the year ended Dec. 31, 2017, filed with the SEC on Feb. 23, 2018. Due to the seasonality of PSCo’s electric and natural gas sales, interim results are not necessarily an appropriate base from which to project annual results.

1. Summary of Significant Accounting Policies

The significant accounting policies set forth in Note 1 to the consolidated financial statements in the PSCo Annual Report on Form 10-K for the year ended Dec. 31, 2017, appropriately represent, in all material respects, the current status of accounting policies and are incorporated herein by reference.

2. Accounting Pronouncements

Recently Issued

Leases —In February 2016, the Financial Accounting Standards Board (FASB) issued Leases, Topic 842 (Accounting Standards Update (ASU) No. 2016-02), which for lessees requires balance sheet recognition of right-of-use assets and lease liabilities for most leases. This guidance will be effective for interim and annual reporting periods beginning after Dec. 15, 2018. PSCo has not yet fully determined the impacts of implementation. However, adoption is expected to occur on Jan. 1, 2019 utilizing the practical expedients provided by the standard and proposed in Targeted Improvements, Topic 842 (Proposed ASU 2018-200). On Jan. 1, 2019 agreements considered leases for the use of office space, equipment and natural gas storage assets, as well as certain purchased power agreements (PPAs) for fossil-fueled generating facilities are expected to be recognized on the consolidated balance sheet.

Recently Adopted

Revenue Recognition— In May 2014, the FASB issued Revenue from Contracts with Customers, Topic 606 (ASU No. 2014-09), which provides a new framework for the recognition of revenue. PSCo implemented the guidance on a modified retrospective basis on Jan. 1, 2018. Results for reporting periods beginning after Dec. 31, 2017 are presented in accordance with Topic 606, while prior period results have not been adjusted and continue to be reported in accordance with prior accounting guidance. Other than increased disclosures regarding revenues related to contracts with customers, the implementation did not have a significant impact on PSCo’s consolidated financial statements. For related disclosures, see Note 13 to the consolidated financial statements.

Classification and Measurement of Financial Instruments — In January 2016, the FASB issued Recognition and Measurement of Financial Assets and Financial Liabilities, Subtopic 825-10 (ASU No. 2016-01), which eliminated the available-for-sale classification for marketable equity securities and also replaced the cost method of accounting for non-marketable equity securities with a model for recognizing impairments and observable price changes. Under the new standard, other than when the consolidation or equity method of accounting is utilized, changes in the fair value of equity securities are recognized in earnings. PSCo implemented the guidance on Jan. 1, 2018 and the implementation did not have a material impact on its consolidated financial statements.

7

Presentation of Net Periodic Benefit Cost —In March 2017, the FASB issued Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, Topic 715 (ASU No. 2017-07), which establishes that only the service cost element of pension cost may be presented as a component of operating income in the income statement. Also under the guidance, only the service cost component of pension cost is eligible for capitalization. As a result of the application of accounting principles for rate regulated entities, a similar amount of pension cost, including non-service components, will be recognized consistent with the historical ratemaking treatment, and the impacts of adoption will be limited to changes in classification of non-service costs in the consolidated statement of income. PSCo implemented the new guidance on Jan. 1, 2018, and as a result, $1.0 million of pension costs were retrospectively reclassified from operating and maintenance expenses to other income, net on the consolidated income statement for the six months ended June 30, 2017. Under a practical expedient permitted by the standard, PSCo used benefit cost amounts disclosed for prior periods as the basis for retrospective application.

3. Selected Balance Sheet Data

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Accounts receivable, net
Accounts receivable	$	290,832		$	314,009
Less allowance for bad debts	(18,539		)	(19,606		)
	$	272,293		$	294,403

(Thousands of Dollars)	June 30, 2018		Dec. 31, 2017
Inventories
Materials and supplies	$	70,940	$	68,940
Fuel	62,532		73,893
Natural gas	36,248		71,656
	$	169,720	$	214,489

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Property, plant and equipment, net
Electric plant	$	12,780,963		$	12,627,592
Natural gas plant	4,211,977			4,102,075
Common and other property	1,040,638			1,022,333
Plant to be retired (a)	10,306			10,949
Construction work in progress	1,444,140			1,014,338
Total property, plant and equipment	19,488,024			18,777,287
Less accumulated depreciation	(4,946,946		)	(4,751,536		)
	$	14,541,078		$	14,025,751

^(a)  In the third quarter of 2017, PSCo early retired Valmont Unit 5 and converted Cherokee Unit 4 from a coal-fueled generating facility to natural gas. PSCo also expects Craig Unit 1 to be early retired in approximately 2025. Amounts are presented net of accumulated depreciation.

4. Income Taxes

Except to the extent noted below, Note 7 to the consolidated financial statements included in PSCo’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 appropriately represents, in all material respects, the current status of other income tax matters, and are incorporated herein by reference.

8

Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences:

	Six Months Ended June 30
	2018		2017
Federal statutory rate	21.0	%	35.0	%
State tax, net of federal tax effect	3.7		3.0
Increases (decreases) in tax from:
Regulatory differences - ARAM (a)	(3.2	)	(0.1	)
Regulatory differences - ARAM deferral (b)	3.0		—
Regulatory differences - other utility plant items	(1.3	)	(0.9	)
Tax credits, net of federal income tax expense	(1.0	)	(0.7	)
Other, net	0.3		0.1
Effective income tax rate	22.5	%	36.4	%

^(a)   The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.

^(b) As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the Tax Cuts and Jobs Act

(TCJA)), the ARAM deferral may decrease during the year, which would result in a reduction to tax expense with a corresponding reduction to revenue.

Federal Audits— PSCO is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows:

Tax Year(s)	Expiration
2009 - 2011	December 2018
2012 - 2014	October 2019
2015	September 2019
2016	September 2020

In 2012, the Internal Revenue Service (IRS) commenced an examination of tax years 2010 and 2011, including the 2009 carryback claim. The IRS proposed an adjustment to the federal tax loss carryback claims and in 2015, the IRS forwarded the issue to the Office of Appeals (Appeals). In 2017 Xcel Energy and Appeals reached an agreement and the benefit related to the agreed upon portions was recognized. PSCo did not accrue any income tax benefit related to this adjustment. In the second quarter of 2018, the Joint Committee on Taxation completed its review and took no exception to the agreement. As a result, the remaining unrecognized tax benefit was released and recorded as a payable to the IRS.

In the third quarter of 2015, the IRS commenced an examination of tax years 2012 and 2013. In the third quarter of 2017, the IRS concluded the audit of tax years 2012 and 2013 and proposed an adjustment that would impact Xcel Energy’s net operating loss (NOL) and effective tax rate (ETR). After evaluating the proposed adjustment Xcel Energy filed a protest with the IRS. As of June 30, 2018, the case has been forwarded to Appeals and Xcel Energy has recognized its best estimate of income tax expense that will result from a final resolution of this issue; however, the outcome and timing of a resolution is unknown.

State Audits — PSCo is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of June 30, 2018, PSCo’s earliest open tax year that is subject to examination by state taxing authorities under applicable statutes of limitations is 2009. There are currently no state income tax audits in progress.

9

Unrecognized Benefits— The unrecognized tax benefit balance includes permanent tax positions, which if recognized would affect the annual ETR. In addition, the unrecognized tax benefit balance includes temporary tax positions for which the ultimate deductibility is highly certain but for which there is uncertainty about the timing of such deductibility. A change in the period of deductibility would not affect the ETR but would accelerate the payment of cash to the taxing authority to an earlier period.

A reconciliation of the amount of unrecognized tax benefit is as follows:

(Millions of Dollars)	June 30, 2018		Dec. 31, 2017
Unrecognized tax benefit — Permanent tax positions	$	4.4	$	4.0
Unrecognized tax benefit — Temporary tax positions	5.0		6.1
Total unrecognized tax benefit	$	9.4	$	10.1

The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows:

(Millions of Dollars)	June 30, 2018			Dec. 31, 2017
NOL and tax credit carryforwards	$	(4.7	)	$	(4.0	)

It is reasonably possible that PSCo’s amount of unrecognized tax benefits could significantly change in the next 12 months as the IRS Appeals progresses and the IRS and state audits resume. As the IRS Appeals progresses and the IRS audit resumes, it is reasonably possible that the amount of unrecognized tax benefit could decrease up to approximately $8 million.

The payable for interest related to unrecognized tax benefits is partially offset by the interest benefit associated with NOL and tax credit carryforwards. The payables for interest related to unrecognized tax benefits at June 30, 2018, and Dec. 31, 2017 were not material. No amounts were accrued for penalties related to unrecognized tax benefits as of June 30, 2018 or Dec. 31, 2017.

5. Rate Matters

Except to the extent noted below, the circumstances set forth in Note 11 to the consolidated financial statements included in PSCo’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Note 5 to the consolidated financial statements to PSCo’s Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018, appropriately represent, in all material respects, the current status of other rate matters, and are incorporated herein by reference.

Tax Reform — Regulatory Proceedings

The specific impacts of the TCJA on customer rates are subject to regulatory approval. Each of the states in Xcel Energy’s service areas, including Colorado, have opened dockets to address the impacts of the TCJA.

Colorado Natural Gas — In February 2018, the administrative law judge (ALJ) approved PSCo and the Colorado Public Utilities Commission (CPUC) Staff’s TCJA settlement agreement which includes a $20 million reduction to provisional rates effective March 1, 2018. A final true-up would provide customers the full net benefit of the TCJA retroactive to January 2018.

Colorado Electric— In April 2018, PSCo, the CPUC Staff and the Office of Consumer Counsel (OCC) filed a TCJA settlement agreement that recommended a customer refund of $42 million in 2018, with the remainder of $59 million be used to accelerate the amortization of an existing prepaid pension asset.  In June 2018, the CPUC approved the customer refund of $42 million, effective June 1, 2018. The CPUC set the decision regarding the remainder of the $59 million for hearing before an ALJ. Revisions to the TCJA settlement will be addressed in a future electric rate case.

10

Pending Regulatory Proceedings — CPUC

Colorado 2017 Multi-Year Electric Rate Case — In October 2017, PSCo filed a multi-year request with the CPUC seeking to increase electric rates approximately $245 million over four years. The request was based on forecast test years (FTY), a 10.0 percent return on equity (ROE) and an equity ratio of 55.25 percent. Interim rates, subject to refund and interest, were to be effective on June 1, 2018.

Revenue Request (Millions of Dollars)	2018		2019		2020		2021		Total
Revenue request	$	74	$	75	$	60	$	36	$	245
Clean Air Clean Jobs Act (CACJA) rider conversion to base rates	90		—		—		—		90
Transmission Cost Adjustment (TCA) rider conversion to base rates	43		—		—		—		43
Total	$	207	$	75	$	60	$	36	$	378
Expected year-end rate base (billions of dollars)	$	6.8	$	7.1	$	7.3	$	7.4

In March 2018, PSCo, CPUC Staff and OCC reached a settlement and filed a motion with the CPUC requesting changes to the procedural schedule and scope of the electric case, which included delaying the implementation of provisional rates from June 2018 to January 2019 and requiring PSCo to file updated test year information for 2019 through 2021 which included the impacts of TCJA. In April 2018, the CPUC denied the motion on procedural grounds and dismissed the electric rate case.

Colorado 2017 Multi-Year Natural Gas Rate Case — In June 2017, PSCo filed a multi-year request with the CPUC seeking to increase retail natural gas rates approximately $139 million over three years. The request, detailed below, was based on FTYs, a 10.0 percent ROE and an equity ratio of 55.25 percent.

Revenue Request (Millions of Dollars)	2018		2019		2020		Total
Revenue request	$	63	$	33	$	43	$	139
Pipeline System Integrity Adjustment (PSIA) rider conversion to base rates (a)	—		94		—		94
Total	$	63	$	127	$	43	$	233
Expected year-end rate base (billions of dollars) (b)	$	1.5	$	2.3	$	2.4

^(a)   The roll-in of PSIA rider revenue into base rates will not have an impact on customer bills or revenue as these costs are already being recovered through the rider. The recovery of incremental PSIA related investments in 2019 and 2020 are included in the base rate request.

^(b)   The additional rate base in 2019 predominantly reflects the roll-in of capital associated with the PSIA rider.

In February 2018, the administrative law judge (ALJ) approved a TCJA settlement agreement between PSCo and the CPUC Staff, which reduced provisional rates by $20 million, based on a preliminary TCJA estimate of $29 million. The settlement remains subject to CPUC approval. The impact of the TCJA will be trued-up later in 2018. Annualized provisional rates of approximately $43 million were effective March 1, 2018.

In May 2018, the ALJ issued an interim recommended decision which would result in a 2018 overall rate increase of approximately $46 million, prior to the impact of the TCJA. The estimated rate increase reflects a 2016 HTY with a 13-month average rate base of $1.6 billion, a ROE of 9.35 percent and an equity ratio of 54.2 percent.

On July 12, 2018, the CPUC deliberated and approved several of the ALJ’s recommendations including application of a 2016 HTY, with a 13-month average rate base, and an ROE of 9.35 percent.  The CPUC adjusted the equity ratio to 54.6 percent and provided no return on the prepaid pension and retiree medical asset.  With these adjustments the total rate increase, prior to TCJA impacts, would be $47 million.

11

The estimated impact of the CPUC’s decision is presented below:

(Millions of Dollars)	Estimated Impact of the CPUC’s Decision
Filed 2018 revenue request based on a FTY	$	63
Impact of the change in test year	5
PSCo’s deficiency based on a 2016 HTY - year-end rate base	68
Adjustments:
ROE at 9.35 percent	(9		)
Equity ratio of 54.6 percent	(2		)
Change in amortization period for certain regulatory assets, including a debt return	(6		)
Loss of return on prepaid pension and retiree medical	(4		)
Change from 2016 year-end to average rate base	(5		)
Other, net	5
Total adjustments	(21		)
Total rate increase, prior to the TCJA impacts	$	47

The CPUC is expected to issue its order on the natural gas rate case in the third quarter of 2018. The CPUC is expected to issue a final decision with the impacts of the TCJA, later in 2018.

Provisional rates, subject to refund, were implemented on Jan. 1, 2018. A current liability which represents PSCo’s best estimate of a refund obligation associated with provisional rates was recorded as of June 30, 2018.

PSIA Rider

In June 2018, PSCo filed for an extension to the PSIA rider through 2020. PSCo requested an expedited decision by Nov. 15, 2018. PSCo also requested authorization to roll-in recovery of costs in the current PSIA rider into base rates effective Jan. 1, 2019, if the CPUC rejects the proposed PSIA extension or fails to rule on the request by the end of 2018.

Additionally, PSCo reduced PSIA revenues by approximately $8 million for 2018 for the impact of the TCJA, effective May 1, 2018. PSIA revenues are subject to the CPUC approved PSIA rider true-up process.

6. Commitments and Contingencies

Except to the extent noted below and in Note 5 to the consolidated financial statements, the circumstances set forth in Notes 11 and 12 to the consolidated financial statements included in PSCo’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Notes 5 and 6 to PSCo’s Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018, appropriately represent, in all material respects, the current status of commitments and contingent liabilities and are incorporated herein by reference. The following include commitments, contingencies and unresolved contingencies that are material to PSCo’s financial position.

PPAs

PSCo purchases power from independent power producing entities that own natural gas fueled power plants for which PSCo is required to reimburse natural gas fuel costs, or to participate in tolling arrangements under which PSCo procures the natural gas required to produce the energy that it purchases. These specific PPAs create a variable interest in the associated independent power producing entity.

PSCo had approximately 1,571 Megawatts (MW) of capacity under long-term PPAs as of June 30, 2018 and Dec. 31, 2017, with entities that have been determined to be variable interest entities. PSCo has concluded that these entities are not required to be consolidated in its consolidated financial statements because it does not have the power to direct the activities that most significantly impact the entities’ economic performance. These agreements have various expiration dates through 2032.

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Environmental Contingencies

Manufactured Gas Plant (MGP), Landfill or Disposal Sites — PSCo is currently involved in investigating and/or remediating several MGP, landfill or other disposal sites. PSCo has identified four sites where contamination is present and where investigation and/or remediation activities are currently underway. Other parties may have responsibility for some portion of the investigation and/or remediation activities. PSCo anticipates that these investigation or remediation activities will continue through at least 2018. PSCo accrued $2 million as of June 30, 2018 and an immaterial amount as of Dec. 31, 2017, for these sites. There may be insurance recovery and/or recovery from other potentially responsible parties that will offset any costs incurred. PSCo anticipates that any amounts spent will be fully recovered from customers.

Environmental Requirements

Air

Revisions to the National Ambient Air Quality Standard (NAAQS) for Ozone - In 2015, the EPA revised the NAAQS for ozone by lowering the eight-hour standard from 75 parts per billion (ppb) to 70 ppb. PSCo meets the 2015 ozone standard in all areas where its generating units operate, except for the Denver Metropolitan Area. PSCo’s retirement of its coal fired plants in the Denver non-attainment area helped Colorado’s plan to mitigate non-attainment. In June 2018, the EPA designated the parts of the Denver Metropolitan Area that currently do not attain the 2008 ozone standards as also not attaining the more stringent 2015 ozone standard. Colorado will continue to consider further reductions that are available in the non-attainment area as it develops plans to meet the ozone standards. The gas plants that operate in PSCo’s non-attainment area may be required to improve or add controls, implement further work practices and/or implement enhanced emissions monitoring as part of future Colorado state plans.

Legal Contingencies

PSCo is involved in various litigation matters that are being defended and handled in the ordinary course of business. The assessment of whether a loss is probable or is a reasonable possibility, and whether the loss or a range of loss is estimable, often involves a series of complex judgments about future events. Management maintains accruals for such losses that are probable of being incurred and subject to reasonable estimation. Management is sometimes unable to estimate an amount or range of a reasonably possible loss in certain situations, including but not limited to when (1) the damages sought are indeterminate, (2) the proceedings are in the early stages, or (3) the matters involve novel or unsettled legal theories. In such cases, there is considerable uncertainty regarding the timing or ultimate resolution of such matters, including a possible eventual loss. For current proceedings not specifically reported herein, management does not anticipate that the ultimate liabilities, if any, arising from such current proceedings would have a material effect on PSCo’s financial statements. Unless otherwise required by GAAP, legal fees are expensed as incurred.

Employment, Tort and Commercial Litigation

Line Extension Disputes — In December 2015, Development Recovery Company (DRC) filed a lawsuit in the Denver District Court, stating PSCo failed to award proper allowances and refunds for line extensions to new developments pursuant to the terms of electric and gas service agreements entered into by PSCo and various developers. The dispute involved claims by over fifty developers. In February 2018, the Colorado Supreme Court denied DRC’s petition to appeal the Denver District Court’s dismissal of the lawsuit, effectively terminating this litigation. However, in January 2018, DRC filed a new lawsuit in Boulder County District Court, asserting a single claim that PSCo was required to file its line extension agreements with the CPUC but failed to do so. This claim is substantially similar to the arguments previously raised by DRC. PSCo filed a motion to dismiss this claim, which was granted in May 2018. DRC subsequently filed an appeal to the Colorado Court of Appeals. It is uncertain when a decision will be rendered regarding this appeal.

PSCo has concluded that a loss is remote with respect to this matter as the service agreements were developed to implement CPUC approved tariffs and PSCo has complied with the tariff provisions. Also, if a loss were sustained, PSCo believes it would be allowed to recover these costs through traditional regulatory mechanisms. The amount or range in dispute is presently unknown and no accrual has been recorded for this matter.

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7. Borrowings and Other Financing Instruments

Short-Term Borrowings

Money Pool — Xcel Energy Inc. and its utility subsidiaries have established a money pool arrangement that allows for short-term investments in and borrowings between the utility subsidiaries. Xcel Energy Inc. may make investments in the utility subsidiaries at market-based interest rates; however, the money pool arrangement does not allow the utility subsidiaries to make investments in Xcel Energy Inc. Money pool borrowings for PSCo were as follows:

(Amounts in Millions, Except Interest Rates)	Three Months Ended June 30, 2018			Year Ended Dec. 31, 2017
Borrowing limit	$	250		$	250
Amount outstanding at period end	—			—
Average amount outstanding	60			—
Maximum amount outstanding	156			20
Weighted average interest rate, computed on a daily basis	1.84		%	0.92		%
Weighted average interest rate at period end	N/A			N/A

Commercial Paper — PSCo meets its short-term liquidity requirements primarily through the issuance of commercial paper and borrowings under its credit facility and the money pool. Commercial paper outstanding for PSCo was as follows:

(Amounts in Millions, Except Interest Rates)	Three Months Ended June 30, 2018			Year Ended Dec. 31, 2017
Borrowing limit	$	700		$	700
Amount outstanding at period end	—			—
Average amount outstanding	84			54
Maximum amount outstanding	257			268
Weighted average interest rate, computed on a daily basis	2.17		%	1.08		%
Weighted average interest rate at period end	N/A			N/A

Letters of Credit— PSCo uses letters of credit, generally with terms of one year, to provide financial guarantees for certain operating obligations. At June 30, 2018 and Dec. 31, 2017, there were $4 million and $3 million, respectively, of letters of credit outstanding under the credit facility. The contract amounts of these letters of credit approximate their fair value and are subject to fees.

Credit Facility — In order to use its commercial paper program to fulfill short-term funding needs, PSCo must have a revolving credit facility in place at least equal to the amount of its commercial paper borrowing limit and cannot issue commercial paper in an aggregate amount exceeding available capacity under this credit facility. The credit facility provides short-term financing in the form of notes payable to banks, letters of credit and back-up support for commercial paper borrowings.

At June 30, 2018, PSCo had the following committed credit facility available (in millions of dollars):

Credit Facility (a)		Drawn(b)		Available
$	700	$	4	$	696

^(a)    This credit facility expires in June 2021.

^(b)    Includes outstanding commercial paper and letters of credit.

All credit facility bank borrowings, outstanding letters of credit and outstanding commercial paper reduce the available capacity under the credit facility. PSCo had no direct advances on the credit facility outstanding at June 30, 2018 and Dec. 31, 2017.

Long-Term Borrowings

PSCo issued $350 million of 3.70 percent first mortgage green bonds due June 15, 2028 and $350 million of 4.10 percent first mortgage green bonds due June 15, 2048.

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8. Fair Value of Financial Assets and Liabilities

Fair Value Measurements

The accounting guidance for fair value measurements and disclosures provides a single definition of fair value and requires certain disclosures about assets and liabilities measured at fair value. A hierarchical framework for disclosing the observability of the inputs utilized in measuring assets and liabilities at fair value is established by this guidance. The three levels in the hierarchy are as follows:

Level 1 — Quoted prices are available in active markets for identical assets or liabilities as of the reporting date. The types of assets and liabilities included in Level 1 are highly liquid and actively traded instruments with quoted prices.

Level 2 — Pricing inputs are other than quoted prices in active markets, but are either directly or indirectly observable as of the reporting date. The types of assets and liabilities included in Level 2 are typically either comparable to actively traded securities or contracts, or priced with models using highly observable inputs.

Level 3 — Significant inputs to pricing have little or no observability as of the reporting date. The types of assets and liabilities included in Level 3 are those valued with models requiring significant management judgment or estimation.

Specific valuation methods include the following:

Cash equivalents — The fair values of cash equivalents are generally based on cost plus accrued interest; money market funds are measured using quoted net asset value.

Interest rate derivatives— The fair values of interest rate derivatives are based on broker quotes that utilize current market interest rate forecasts.

Commodity derivatives— The methods used to measure the fair value of commodity derivative forwards and options utilize forward prices and volatilities, as well as pricing adjustments for specific delivery locations, and are generally assigned a Level 2 classification. When contractual settlements extend to periods beyond those readily observable on active exchanges or quoted by brokers, the significance of the use of less observable forecasts of long-term forward prices and volatilities on a valuation is evaluated, and may result in Level 3 classification.

Derivative Instruments Fair Value Measurements

PSCo enters into derivative instruments, including forward contracts, futures, swaps and options, for trading purposes and to manage risk in connection with changes in interest rates, utility commodity prices and vehicle fuel prices.

Interest Rate Derivatives — PSCo enters into various instruments that effectively fix the interest payments on certain floating rate debt obligations or effectively fix the yield or price on a specified benchmark interest rate for an anticipated debt issuance for a specific period. These derivative instruments are generally designated as cash flow hedges for accounting purposes.

At June 30, 2018, accumulated other comprehensive losses related to interest rate derivatives included $1.2 million of net losses expected to be reclassified into earnings during the next 12 months as the related hedged interest rate transactions impact earnings, including forecasted amounts for unsettled hedges, as applicable.

Wholesale and Commodity Trading Risk — PSCo conducts various wholesale and commodity trading activities, including the purchase and sale of electric capacity, energy, energy-related instruments and natural gas related instruments, including derivatives. PSCo’s risk management policy allows management to conduct these activities within guidelines and limitations as approved by its risk management committee, which is made up of management personnel not directly involved in the activities governed by this policy.

Commodity Derivatives — PSCo enters into derivative instruments to manage variability of future cash flows from changes in commodity prices in its electric and natural gas operations, as well as for trading purposes. This could include the purchase or sale of energy or energy-related products, natural gas to generate electric energy, natural gas for resale, and vehicle fuel.

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PSCo enters into derivative instruments that mitigate commodity price risk on behalf of electric and natural gas customers but may not be designated as qualifying hedging transactions. Changes in the fair value of non-trading commodity derivative instruments are recorded in other comprehensive income or deferred as a regulatory asset or liability. The classification as a regulatory asset or liability is based on commission approved regulatory recovery mechanisms. PSCo had no income related to the ineffectiveness of cash flow hedges for the three and six months ended June 30, 2018 and 2017.

Additionally, PSCo enters into commodity derivative instruments for trading purposes not directly related to commodity price risks associated with serving its electric and natural gas customers. Changes in the fair value of these commodity derivatives are recorded in electric operating revenues, net of amounts credited to customers under margin-sharing mechanisms.

The following table details the gross notional amounts of commodity forwards and options at June 30, 2018 and Dec. 31, 2017:

(Amounts in Thousands)(a)(b)	June 30, 2018	Dec. 31, 2017
Megawatt hours of electricity	20,813	22,260
Million British thermal units of natural gas	18,798	13,410

^(a)  Amounts are not reflective of net positions in the underlying commodities.

^(b)  Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

The following tables detail the impact of derivative activity during the three months ended June 30, 2018 and 2017 on accumulated other comprehensive loss, regulatory assets and liabilities, and income:

	Three Months Ended June 30, 2018
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)		Pre-Tax Gains Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	402	(a)	$	—	$	—
Total	$	—	$	—		$	402		$	—	$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—	$	200	(b)
Natural gas commodity	—		(249		)	—			—		—		(c)
Total	$	—	$	(249	)	$	—		$	—	$	200

	Six Months Ended June 30, 2018
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:						Pre-Tax (Gains) Losses Recognized During the Period in Income
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	800	(a)	$	—		$	—
Total	$	—	$	—		$	800		$	—		$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—		$	724		(b)
Natural gas commodity	—		(420		)	—			2,749		(c)	(1,581		)	(c)
Total	$	—	$	(420	)	$	—		$	2,749		$	(857	)

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	Three Months Ended June 30, 2017
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)		Pre-Tax Losses Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	403	(a)	$	—	$	—
Total	$	—	$	—		$	403		$	—	$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—	$	(192	)	(b)
Natural gas commodity	—		(1,621		)	—			—		—			(c)
Total	$	—	$	(1,621	)	$	—		$	—	$	(192	)

	Six Months Ended June 30, 2017
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)			Pre-Tax Gains (Losses) Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	801	(a)	$	—		$	—
Total	$	—	$	—		$	801		$	—		$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—		$	187		(b)
Natural gas commodity	—		(7,008		)	—			282		(c)	(2,990		)	(c)
Total	$	—	$	(7,008	)	$	—		$	282		$	(2,803	)

^(a)  Amounts are recorded to interest charges.

^(b)  Amounts are recorded to interest charges. Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue as appropriate.

^(c)  Certain derivatives are utilized to mitigate natural gas price risk for electric generation and are recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. Amounts for the three and six months endedJune 30, 2018 included no settlement gains or losses and $1.2 million of settlement losses, respectively. Amounts for the three and six months ended June 30, 2017 included no settlement gains or losses and $0.9 million of settlement gains, respectively. The remaining derivative settlement gains and losses for the six months endedJune 30, 2018 and 2017 relate to natural gas operations and are recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

PSCo had no derivative instruments designated as fair value hedges during the three and six months ended June 30, 2018 and 2017. Therefore, no gains or losses from fair value hedges or related hedged transactions were recognized for these periods.

Consideration of Credit Risk and Concentrations — PSCo continuously monitors the creditworthiness of the counterparties to its interest rate derivatives and commodity derivative contracts prior to settlement, and assesses each counterparty’s ability to perform on the transactions set forth in the contracts. Given this assessment, as well as an assessment of the impact of PSCo’s own credit risk when determining the fair value of derivative liabilities, the impact of credit risk was immaterial to the fair value of unsettled commodity derivatives presented in the consolidated balance sheets.

PSCo employs additional credit risk control mechanisms when appropriate, such as letters of credit, parental guarantees, standardized master netting agreements and termination provisions that allow for offsetting of positive and negative exposures. Credit exposure is monitored and, when necessary, the activity with a specific counterparty is limited until credit enhancement is provided.

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PSCo’s most significant concentrations of credit risk with particular entities or industries are contracts with counterparties to its wholesale, trading and non-trading commodity activities. At June 30, 2018, four of PSCo’s 10 most significant counterparties for these activities, comprising $27.6 million or 47 percent of this credit exposure, had investment grade credit ratings from Standard & Poor’s, Moody’s or Fitch Ratings. Five of the 10 most significant counterparties, comprising $12.2 million or 21 percent of this credit exposure, were not rated by these external agencies, but based on PSCo’s internal analysis, had credit quality consistent with investment grade. The one remaining significant counterparty, comprising $5.2 million or 9 percent of this credit exposure, had credit quality less than investment grade, based on ratings from external analysis. Nine of these significant counterparties are municipal or cooperative electric entities, or other utilities.

Credit Related Contingent Features—Contract provisions for derivative instruments that PSCo enters into, including those accounted for as normal purchase-normal sale contracts and therefore not reflected on the balance sheet, may require the posting of collateral or settlement of the contracts for various reasons, including if PSCo’s credit ratings are downgraded below its investment grade credit rating by any of the major credit rating agencies or for cross-default contractual provisions that could result in the settlement of such contracts if there was a failure under other financing arrangements related to payment terms or other covenants. At June 30, 2018 and Dec. 31, 2017, there were no derivative instruments in a material liability position with such underlying contract provisions.

Certain derivative instruments are also subject to contract provisions that contain adequate assurance clauses. These provisions allow counterparties to seek performance assurance, including cash collateral, in the event that PSCo’s ability to fulfill its contractual obligations is reasonably expected to be impaired. PSCo had no collateral posted related to adequate assurance clauses in derivative contracts as of June 30, 2018 and Dec. 31, 2017.

Recurring Fair Value Measurements—The following table presents, for each of the fair value hierarchy levels, PSCo’s assets and liabilities measured at fair value on a recurring basis at June 30, 2018:

	June 30, 2018
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative assets
Other derivative instruments:
Commodity trading	$	332	$	12,812	$	118	$	13,262	$	(9,729	)	$	3,533
Natural gas commodity	—		919		—		919		—			919
Total current derivative assets	$	332	$	13,731	$	118	$	14,181	$	(9,729	)	4,452
PPAs (a)												888
Current derivative instruments												$	5,340
Noncurrent derivative assets
Other derivative instruments:
Commodity trading	$	1	$	2,715	$	—	$	2,716	$	—		$	2,716
Total noncurrent derivative assets	$	1	$	2,715	$	—	$	2,716	$	—		2,716
PPAs (a)												—
Noncurrent derivative instruments												$	2,716

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	June 30, 2018
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative liabilities
Other derivative instruments:
Commodity trading	$	333	$	12,409	$	—	$	12,742	$	(12,474	)	$	268
Total current derivative liabilities	$	333	$	12,409	$	—	$	12,742	$	(12,474	)	268
PPAs (a)												5,160
Current derivative instruments												$	5,428
Noncurrent derivative liabilities
Other derivative instruments:
Commodity trading	$	—	$	2,382	$	—	$	2,382	$	—		$	2,382
Total noncurrent derivative liabilities	$	—	$	2,382	$	—	$	2,382	$	—		2,382
PPAs (a)												89
Noncurrent derivative instruments												$	2,471

^(a)  During 2006, PSCo qualified these contracts under the normal purchase exception. Based onthis qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

^(b)  PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at June 30, 2018. At June 30, 2018, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $2.7 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

The following table presents, for each of the fair value hierarchy levels, PSCo’s assets and liabilities measured at fair value on a recurring basis at Dec. 31, 2017:

	Dec. 31, 2017
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative assets
Other derivative instruments:
Commodity trading	$	528	$	4,488	$	12	$	5,028	$	(3,554	)	$	1,474
Natural gas commodity	—		18		—		18		(10		)	8
Total current derivative assets	$	528	$	4,506	$	12	$	5,046	$	(3,564	)	1,482
PPAs (a)												1,715
Current derivative instruments												$	3,197
Noncurrent derivative assets
Other derivative instruments:
Commodity trading	$	—	$	1,541	$	—	$	1,541	$	(563	)	$	978
Total noncurrent derivative assets	$	—	$	1,541	$	—	$	1,541	$	(563	)	978
PPAs (a)												31
Noncurrent derivative instruments												$	1,009

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	Dec. 31, 2017
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative liabilities
Other derivative instruments:
Commodity trading	$	446	$	4,285	$	6	$	4,737	$	(3,431	)	$	1,306
Natural gas commodity	—		1,016		—		1,016		(10		)	1,006
Total current derivative liabilities	$	446	$	5,301	$	6	$	5,753	$	(3,441	)	2,312
PPAs (a)												5,036
Current derivative instruments												$	7,348
Noncurrent derivative liabilities
Commodity trading	$	—	$	1,362	$	—	$	1,362	$	(563	)	$	799
Total noncurrent derivative liabilities	$	—	$	1,362	$	—	$	1,362	$	(563	)	799
PPAs (a)												$	2,669
Noncurrent derivative instruments												$	3,468

^(a)  During 2006, PSCo qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

^(b)  PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

There were $0.1 million of gains recognized in earnings for Level 3 commodity trading derivatives in the three and six months ended June 30, 2018. There were immaterial gains and losses recognized in earnings for Level 3 commodity trading derivatives in the three and six months ended June 30, 2017.

PSCo recognizes transfers between levels as of the beginning of each period. There were no transfers of amounts between levels for derivative instruments for the three and six months ended June 30, 2018 and 2017.

Fair Value of Long-Term Debt

As of June 30, 2018 and Dec. 31, 2017, other financial instruments for which the carrying amount did not equal fair value were as follows:

	June 30, 2018				Dec. 31, 2017
(Thousands of Dollars)	Carrying Amount		Fair Value		Carrying Amount		Fair Value
Long-term debt, including current portion	$	5,300,062	$	5,464,543	$	4,608,275	$	5,024,840

The fair value of PSCo’s long-term debt is estimated based on recent trades and observable spreads from benchmark interest rates for similar securities. The fair value estimates are based on information available to management as of June 30, 2018 and Dec. 31, 2017, and given the observability of the inputs to these estimates, the fair values presented for long-term debt have been assigned a Level 2.

9. Other Income, Net

Other income, net consisted of the following:

	Three Months Ended June 30						Six Months Ended June 30
(Thousands of Dollars)	2018			2017			2018			2017
Interest income	$	508		$	609		$	372		$	984
Other nonoperating income	483			1,326			956			4,747
Insurance policy expense	(73		)	(103		)	(150		)	(182		)
Benefits non-service cost	(119		)	(513		)	(148		)	(1,026		)
Other income, net	$	799		$	1,319		$	1,030		$	4,523

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10. Segment Information

Operating results from the regulated electric utility and regulated natural gas utility are each separately and regularly reviewed by PSCo’s chief operating decision maker. PSCo evaluates performance based on profit or loss generated from the product or service provided. These segments are managed separately because the revenue streams are dependent upon regulated rate recovery, which is separately determined for each segment.

PSCo has the following reportable segments: regulated electric utility, regulated natural gas utility and all other.

• PSCo’s regulated electric utility segment generates, transmits and distributes electricity primarily in portions of Colorado. In addition, this segment includes sales for resale and provides wholesale transmission service to various entities in the United States. Regulated electric utility also includes PSCo’s commodity trading operations.

• PSCo’s regulated natural gas utility segment transports, stores and distributes natural gas primarily in portions of Colorado.

• Revenues from operating segments not included above are below the necessary quantitative thresholds and are therefore included in the all other category. Those primarily include steam revenue, appliance repair services and nonutility real estate activities.

Asset and capital expenditure information is not provided for PSCo’s reportable segments because as an integrated electric and natural gas utility, PSCo operates significant assets that are not dedicated to a specific business segment, and reporting assets and capital expenditures by business segment would require arbitrary and potentially misleading allocations which may not necessarily reflect the assets that would be required for the operation of the business segments on a stand-alone basis.

To report income from operations for regulated electric and regulated natural gas utility segments, the majority of costs are directly assigned to each segment. However, some costs, such as common depreciation, common operating and maintenance (O&M) expenses and interest expense are allocated based on cost causation allocators. A general allocator is used for certain general and administrative expenses, including office supplies, rent, property insurance and general advertising.

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other			Reconciling Eliminations			Consolidated Total
Three Months Ended June 30, 2018
Operating revenues (a)(b)	$	716,195	$	186,661	$	9,010		$	—		$	911,866
Intersegment revenues	76		79		—			(155		)	—
Total revenues	$	716,271	$	186,740	$	9,010		$	(155	)	$	911,866
Net income (loss)	$	101,956	$	20,963	$	(623	)	$	—		$	122,296

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Reconciling Eliminations			Consolidated Total
Three Months Ended June 30, 2017
Operating revenues (a)(b)	$	729,920	$	192,777	$	8,219	$	—		$	930,916
Intersegment revenues	67		40		—		(107		)	—
Total revenues	$	729,987	$	192,817	$	8,219	$	(107	)	$	930,916
Net income	$	87,403	$	12,835	$	349	$	—		$	100,587

^(a)    Operating revenues include $0 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017.

^(b)    Operating revenues include $1 million of other affiliate revenue for the three months ended June 30, 2018 and 2017.

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other			Reconciling Eliminations			Consolidated Total
Six Months Ended June 30, 2018
Operating revenues from external customers	$	1,414,469	$	550,647	$	20,048		$	—		$	1,985,164
Intersegment revenues	188		143		—			(331		)	—
Total revenues	$	1,414,657	$	550,790	$	20,048		$	(331	)	$	1,985,164
Net income (loss)	$	181,507	$	74,675	$	(164	)	$	—		$	256,018

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(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Reconciling Eliminations			Consolidated Total
Six Months Ended June 30, 2017
Operating revenues from external customers	$	1,441,308	$	548,913	$	21,229	$	—		$	2,011,450
Intersegment revenues	159		96		—		(255		)	—
Total revenues	$	1,441,467	$	549,009	$	21,229	$	(255	)	$	2,011,450
Net income	$	163,547	$	47,318	$	1,268	$	—		$	212,133

^(a)    Operating revenues include $0 million and $1 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017.

^(b)    Operating revenues include $2 million of other affiliate revenue for the six months ended June 30, 2018 and 2017.

11. Benefit Plans and Other Postretirement Benefits

Components of Net Periodic Benefit Cost (Credit)

	Three Months Ended June 30
	2018			2017			2018			2017
(Thousands of Dollars)	Pension Benefits						Postretirement Health Care Benefits
Service cost	$	7,271		$	6,820		$	152		$	192
Interest cost (a)	11,814			12,640			3,748			4,191
Expected return on plan assets (a)	(17,130		)	(17,134		)	(5,674		)	(5,476		)
Amortization of prior service credit (a)	(845		)	(803		)	(1,544		)	(1,562		)
Amortization of net loss (a)	7,815			7,089			1,021			961
Net periodic benefit cost (credit)	8,925			8,612			(2,297		)	(1,694		)
Credits not recognized due to the effects of regulation	895			426			—			—
Net benefit cost (credit) recognized for financial reporting	$	9,820		$	9,038		$	(2,297	)	$	(1,694	)

	Six Months Ended June 30
	2018			2017			2018			2017
(Thousands of Dollars)	Pension Benefits						Postretirement Health Care Benefits
Service cost	$	14,542		$	13,640		$	304		$	384
Interest cost (a)	23,628			25,280			7,497			8,382
Expected return on plan assets (a)	(34,260		)	(34,268		)	(11,349		)	(10,952		)
Amortization of prior service credit (a)	(1,690		)	(1,606		)	(3,089		)	(3,124		)
Amortization of net loss (a)	15,630			14,178			2,042			1,922
Net periodic benefit cost (credit)	17,850			17,224			(4,595		)	(3,388		)
Credits not recognized due to the effects of regulation	2,370			1,162			—			—
Net benefit cost (credit) recognized for financial reporting	$	20,220		$	18,386		$	(4,595	)	$	(3,388	)

^(a)   The components of net periodic cost other than the service cost component are included in the line item “other income, net” in the income statement or capitalized on the balance sheet as a regulatory asset.

In January 2018, contributions of $150 million were made across four of Xcel Energy’s pension plans, of which $22.0 million was attributable to PSCo. Xcel Energy does not expect additional pension contributions during 2018.

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12. Other Comprehensive Income (Loss)

Changes in accumulated other comprehensive loss, net of tax, for the three and six months ended June 30, 2018 and 2017 were as follows:

	Three Months Ended June 30, 2018
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at April 1	$	(26,165	)	$	(265	)	$	(26,430	)
Losses reclassified from net accumulated other comprehensive loss	303			2			305
Net current period other comprehensive income	303			2			305
Accumulated other comprehensive loss at June 30	$	(25,862	)	$	(263	)	$	(26,125	)

	Three Months Ended June 30, 2017
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at April 1	$	(22,534	)	$	(219	)	$	(22,753	)
Losses reclassified from net accumulated other comprehensive loss	250			1			251
Net current period other comprehensive income	250			1			251
Accumulated other comprehensive loss at June 30	$	(22,284	)	$	(218	)	$	(22,502	)

	Six Months Ended June 30, 2018
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at Jan. 1	$	(26,465	)	$	(267	)	$	(26,732	)
Losses reclassified from net accumulated other comprehensive loss	603			4			607
Net current period other comprehensive income	603			4			607
Accumulated other comprehensive loss at June 30	$	(25,862	)	$	(263	)	$	(26,125	)

	Six Months Ended June 30, 2017
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at Jan. 1	$	(22,780	)	$	(220	)	$	(23,000	)
Losses reclassified from net accumulated other comprehensive loss	496			2			498
Net current period other comprehensive income	496			2			498
Accumulated other comprehensive loss at June 30	$	(22,284	)	$	(218	)	$	(22,502	)

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Reclassifications from accumulated other comprehensive loss for the three and six months ended June 30, 2018 and 2017 were as follows:

	Amounts Reclassified from Accumulated Other Comprehensive Loss
(Thousands of Dollars)	Three Months Ended June 30, 2018				Three Months Ended June 30, 2017
Losses on cash flow hedges:
Interest rate derivatives	$	402		(a)	$	403		(a)
Total, pre-tax	402				403
Tax benefit	(99		)		(153		)
Total, net of tax	303				250
Defined benefit pension and postretirement losses:
Amortization of net loss	2			(b)	2			(b)
Total, pre-tax	2				2
Tax benefit	—				(1		)
Total, net of tax	2				1
Total amounts reclassified, net of tax	$	305			$	251

	Amounts Reclassified from Accumulated Other Comprehensive Loss
(Thousands of Dollars)	Six Months Ended June 30, 2018				Six Months Ended June 30, 2017
Losses on cash flow hedges:
Interest rate derivatives	$	800		(a)	$	801		(a)
Total, pre-tax	800				801
Tax benefit	(197		)		(305		)
Total, net of tax	$	603			$	496
Defined benefit pension and postretirement losses:
Amortization of net loss	$	4		(b)	$	4		(b)
Total, pre-tax	4				4
Tax benefit	—				(2		)
Total, net of tax	4				2
Total amounts reclassified, net of tax	$	607			$	498

^(a)  Included in interest charges.

^(b)  Included in the computation of net periodic pension and postretirement benefit costs. See Note 11 to the consolidated financial statements for details regarding these benefit plans.

13. Revenues

PSCo principally generates revenue from the generation, transmission, distribution and sale of electricity and the transportation, distribution and sale of natural gas to wholesale and retail customers. Performance obligations related to the sale of energy are satisfied as energy is delivered to customers. PSCo recognizes revenue in an amount that corresponds directly to the price of the energy delivered to the customer. The measurement of energy sales to customers is generally based on the reading of their meters, which occurs on a systematic basis throughout the month. At the end of each month, amounts of energy delivered to customers since the date of the last meter reading are estimated, and the corresponding unbilled revenue is recognized. Contract terms are generally short-term in nature, and as such PSCo does not recognize a separate financing component of its collections from customers. PSCo presents its revenues net of any excise or other fiduciary-type taxes or fees.

PSCo has various rate-adjustment mechanisms in place that provide for the recovery of natural gas, electric fuel and purchased energy costs. These cost-adjustment tariffs may increase or decrease the level of revenue collected from customers and are revised periodically for differences between the total amount collected under the clauses and the costs incurred. When applicable, under governing regulatory commission rate orders, fuel cost over-recoveries (the excess of fuel revenue billed to customers over fuel costs incurred) are deferred as regulatory liabilities and under-recoveries (the excess of fuel costs incurred over fuel revenues billed to customers) are deferred as regulatory assets.

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Certain rate rider mechanisms qualify as alternative revenue programs under GAAP. These mechanisms arise from costs imposed upon the utility by action of a regulator or legislative body related to an environmental, public safety or other mandate. When certain criteria are met (including collection within 24 months), revenue is recognized equal to the revenue requirement, which may include return on rate base items and incentives. The mechanisms are revised periodically for differences between the total amount collected and the revenue recognized, which may increase or decrease the level of revenue collected from customers. Alternative revenue is recorded on a gross basis and is disclosed separate from revenue from contracts with customers in the period earned.

In the following tables, revenue is classified by the type of goods/services rendered and market/customer type. The tables also reconcile revenue to the reportable segments.

	Three Months Ended June 30, 2018
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	222,687	$	104,000	$	2,560	$	329,247
Commercial and industrial (C&I)	383,707		38,904		5,326		427,937
Other	11,455		—		—		11,455
Total retail	617,849		142,904		7,886		768,639
Wholesale	36,148		—		—		36,148
Transmission	13,159		—		—		13,159
Other	14,696		18,845		—		33,541
Total revenue from contracts with customers	681,852		161,749		7,886		851,487
Alternative revenue and other	34,343		24,912		1,124		60,379
Total revenues	$	716,195	$	186,661	$	9,010	$	911,866

	Three Months Ended June 30, 2017
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	225,921	$	117,393	$	2,639	$	345,953
C&I	392,280		45,032		4,457		441,769
Other	11,711		—		—		11,711
Total retail	629,912		162,425		7,096		799,433
Wholesale	41,579		—		—		41,579
Transmission	13,180		—		—		13,180
Other	16,837		20,040		—		36,877
Total revenue from contracts with customers	701,508		182,465		7,096		891,069
Alternative revenue and other	28,412		10,312		1,123		39,847
Total revenues	$	729,920	$	192,777	$	8,219	$	930,916

	Six Months Ended June 30, 2018
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	450,336	$	331,746	$	5,256	$	787,338
C&I	726,933		124,932		12,485		864,350
Other	23,631		—		60		23,691
Total retail	1,200,900		456,678		17,801		1,675,379
Wholesale	84,038		—		—		84,038
Transmission	25,411		—		—		25,411
Other	33,527		43,774		—		77,301
Total revenue from contracts with customers	1,343,876		500,452		17,801		1,862,129
Alternative revenue and other	70,593		50,195		2,247		123,035
Total revenues	$	1,414,469	$	550,647	$	20,048	$	1,985,164

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	Six Months Ended June 30, 2017
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	460,355	$	343,303	$	5,203	$	808,861
C&I	751,977		131,274		13,717		896,968
Other	24,386		—		63		24,449
Total retail	1,236,718		474,577		18,983		1,730,278
Wholesale	85,155		—		—		85,155
Transmission	27,819		—		—		27,819
Other	33,532		42,650		—		76,182
Total revenue from contracts with customers	1,383,224		517,227		18,983		1,919,434
Alternative revenue and other	58,084		31,686		2,246		92,016
Total revenues	$	1,441,308	$	548,913	$	21,229	$	2,011,450

Item 2 — MANAGEMENT’S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS OF OPERATIONS

Discussion of financial condition and liquidity for PSCo is omitted per conditions set forth in general instructions H (1) (a) and (b) of Form 10-Q for wholly owned subsidiaries. It is replaced with management’s narrative analysis of the results of operations set forth in general instructions H (2) (a) of Form 10-Q for wholly owned subsidiaries (reduced disclosure format).

Financial Review

The following discussion and analysis by management focuses on those factors that had a material effect on PSCo’s financial condition, results of operations and cash flows during the periods presented, or are expected to have a material impact in the future. It should be read in conjunction with the accompanying unaudited consolidated financial statements and related notes to the consolidated financial statements. Due to the seasonality of PSCo’s electric and natural gas sales, such interim results are not necessarily an appropriate base from which to project annual results.

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Forward-Looking Statements

Except for the historical statements contained in this report, the matters discussed herein, are forward-looking statements that are subject to certain risks, uncertainties and assumptions. Such forward-looking statements including the TCJA’s impact to PSCo and its customers, as well as assumptions and other statements identified in this document by the words “anticipate,” “believe,” “could,” “estimate,” “expect,” “intend,” “may,” “objective,” “outlook,” “plan,” “project,” “possible,” “potential,” “should,” “will,” “would” and similar expressions. Actual results may vary materially. Forward-looking statements speak only as of the date they are made, and we expressly disclaim any obligation to update any forward-looking information. The following factors, in addition to those discussed elsewhere in this Quarterly Report on Form 10-Q and in other securities filings (including PSCo’s Annual Report on Form 10-K for the fiscal year ended Dec. 31, 2017 and subsequent securities filings), could cause actual results to differ materially from management expectations as suggested by such forward-looking information: general economic conditions, including inflation rates, monetary fluctuations and their impact on capital expenditures and the ability of PSCo and its subsidiaries to obtain financing on favorable terms; business conditions in the energy industry; including the risk of a slow down in the U.S. economy or delay in growth, recovery, trade, fiscal, taxation and environmental policies in areas where PSCo has a financial interest; customer business conditions; actions of credit rating agencies; competitive factors including the extent and timing of the entry of additional competition in the markets served by PSCo and its subsidiaries; unusual weather; effects of geopolitical events, including war and acts of terrorism; cyber security threats and data security breaches; state, federal and foreign legislative and regulatory initiatives that affect cost and investment recovery, have an impact on rates or have an impact on asset operation or ownership or impose environmental compliance conditions; structures that affect the speed and degree to which competition enters the electric and natural gas markets; costs and other effects of legal and administrative proceedings, settlements, investigations and claims; financial or regulatory accounting policies imposed by regulatory bodies; outcomes of regulatory proceedings; availability or cost of capital; and employee work force factors.

Non-GAAP Financial Measures

The following discussion includes financial information prepared in accordance with GAAP, as well as certain non-GAAP financial measures such as electric margin and natural gas margin.  Generally, a non-GAAP financial measure is a numerical measure of a company’s financial performance, financial position or cash flows that excludes (or includes) amounts that are adjusted from the most directly comparable measure calculated and presented in accordance with GAAP. PSCo’s management uses non-GAAP measures internally for financial planning and analysis, for reporting of results to the Board of Directors and when communicating its earnings outlook to analysts and investors. Non-GAAP financial measures are intended to supplement investors’ understanding of our operating performance and should not be considered alternatives for financial measures presented in accordance with GAAP. These measures are discussed in more detail below and may not be comparable to other companies’ similarly titled non-GAAP financial measures.

Electric margin is presented as electric revenues less electric fuel and purchased power expenses and natural gas margin is presented as natural gas revenues less the cost of natural gas sold and transported. Expenses incurred for electric fuel and purchased power and the cost of natural gas sold and transported are generally recovered through various recovery mechanisms, and as a result, changes in these expenses are offset in operating revenues. Management believes electric and natural gas margins provide the most meaningful basis for evaluating our operations because they exclude the revenue impact of fluctuations in these expenses. These margins can be reconciled to operating income, a GAAP measure, by including steam and other operating revenues, cost of sales - steam and other, O&M expenses, demand side management (DSM) expenses, depreciation and amortization and taxes (other than income taxes).

Results of Operations

PSCo’s net income was approximately $256 million for the second quarter of 2018, compared with approximately $212 million for the same period of 2017. The increase was driven by higher electric and natural gas margins due to the impact of an interim rate increase, subject to refund, and favorable weather and increased allowance for funds used during construction (AFUDC) primarily related to the Rush Creek wind project. These items were partially offset by higher interest charges and depreciation expense.

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Electric Revenues and Margin

Electric revenues and fuel and purchased power expenses are impacted by fluctuations in the price of natural gas and coal used in the generation of electricity. However, these price fluctuations have minimal impact on electric margin due to fuel recovery mechanisms that recover fuel expenses. The following table details the electric revenues and margin:

	Six Months Ended June 30
(Millions of Dollars)	2018			2017
Electric revenues before impact of the TCJA	$	1,460		$	1,441
Electric fuel and purchased power	(553		)	(568		)
Electric margin before impact of the TCJA	$	907		$	873
Impact of the TCJA (offset as a reduction in income tax expense)	(46		)	—
Electric margin	$	861		$	873

The following tables summarize the components of the changes in electric revenues and electric margin for the six months endedJune 30:

Electric Revenues

(Millions of Dollars)	2018 vs. 2017
Fuel and purchased power cost recovery	$	(8	)
Estimated impact of weather	8
DSM program revenues (offset by expenses)	7
Trading	5
DSM incentive	2
Non-fuel riders	2
Firm wholesale	2
Other, net	1
Total increase in electric revenues before impact of the TCJA	$	19
Impact of the TCJA (offset as a reduction in income tax expense)	(46		)
Total decrease in electric revenues	$	(27	)

Electric Margin

(Millions of Dollars)	2018 vs. 2017
Estimated impact of weather	$	8
DSM program revenues (offset by expenses)	7
Trading	3
DSM incentive	2
Fuel handling and procurement	2
Non-fuel riders	2
Firm wholesale	2
Other, net	8
Total increase in electric margin before impact of the TCJA	$	34
Impact of the TCJA (offset as a reduction in income tax expense)	(46		)
Total decrease in electric margin	$	(12	)

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Natural Gas Revenues and Margin

Total natural gas expense varies with changing sales and the cost of natural gas. However, fluctuations in the cost of natural gas has minimal impact on natural gas margin due to natural gas cost recovery mechanisms. The following table details natural gas revenues and margin:

	Six Months Ended June 30
(Millions of Dollars)	2018			2017
Natural gas revenues before impact of the TCJA	$	566		$	549
Cost of natural gas sold and transported	(250		)	(267		)
Natural gas margin before impact of the TCJA	$	316		$	282
Impact of the TCJA (offset as a reduction in income tax expense)	(15		)	—
Natural gas margin	$	301		$	282

The following tables summarize the components of the changes in natural gas revenues and natural gas margin for the six months endedJune 30:

Natural Gas Revenues

(Millions of Dollars)	2018 vs. 2017
Retail rate increase (interim, subject to refund)	$	20
Infrastructure and integrity riders	8
Estimated impact of weather	4
Purchased natural gas adjustment clause recovery	(16		)
Other, net	1
Total increase in natural gas revenues before impact of the TCJA	$	17
Impact of the TCJA (offset as a reduction in income tax expense)	(15		)
Total increase in natural gas revenues	$	2

Natural Gas Margin

(Millions of Dollars)	2018 vs. 2017
Retail rate increase (interim, subject to refund)	$	20
Infrastructure and integrity riders	8
Estimated impact of weather	4
Other, net	2
Total increase in natural gas margin before impact of the TCJA	$	34
Impact of the TCJA (offset as a reduction in income tax expense)	(15		)
Total increase in natural gas margin	$	19

Non-Fuel Operating Expenses and Other Items

DSM Program Expenses — DSM program expenses increased $8 million, or 13.7 percent, for 2018 year-to-date. The increase was due to higher recovery rates for electric and natural gas sales. DSM expenses are generally recovered concurrently through riders and base rates. Timing of recovery may not correspond to the period in which costs were incurred.

Depreciation and Amortization— Depreciation and amortization expense increased $6 million, or 2.4 percent, for 2018 year-to-date. The increase was primarily driven by capital expenditures due to planned system investments.

AFUDC, Equity and Debt— AFUDC increased $19 million for 2018 year-to-date.  The increase was primarily due to the Rush Creek wind project.

Interest Charges— Interest charges increased $9 million, or 9.6 percent, for 2018 year-to-date. The increase was primarily due to higher debt levels to fund capital investments, partially offset by refinancings at lower interest rates.

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Income Taxes — Income tax expense decreased $47 million for the first six months of 2018 compared with the same period in 2017. The decrease was primarily driven by a lower federal tax rate due to the TCJA. The ETR was 22.5 percent for the first six months of 2018 compared with 36.4 percent for the same period of 2017. The lower ETR in 2018 is primarily due to the lower federal tax rate. See Note 4 to the consolidated financial statements.

Public Utility Regulation

Except to the extent noted below and in Note 5 to the consolidated financial statements, the circumstances set forth in Public Utility Regulation included in Item 1 of PSCo’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 and Public Utility Regulation included in Item 2 of PSCo’s Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018, appropriately represent, in all material respects, the current status of public utility regulation and are incorporated herein by reference.

Colorado Energy Plan (CEP) — In 2016, PSCo filed its 2016 Electric Resource Plan (ERP) which included the estimated need for additional generation resources through spring of 2024. In 2017, PSCo filed an updated capacity need with the CPUC of 450 MW in 2023.

In 2017, PSCo and various other stakeholders filed a stipulation agreement proposing the CEP, an alternative plan that increases PSCo’s potential capacity need up to 1,110 MW due to the proposed retirement of two coal units.

In June 2018, PSCo filed its 120-day update report with the CPUC which includes multiple portfolios and recommends a preferred CEP portfolio. PSCo's investment under the preferred CEP portfolio would be approximately $1 billion, including investment in transmission to support the significant increase in renewable generation in the state. The preferred CEP portfolio includes the following additions as well as the retirement of the two coal-fired generation units:

	Total Capacity	PSCo's Ownership
Wind generation	1,100 MW	500 MW
Solar generation	700 MW	—
Battery storage	275 MW	—
Natural gas generation	380 MW	380 MW

On July 13, 2018, the Independent Evaluator (IE) for the ERP filed their report on the process, modeling and evaluation of the various offers received through the RFP process.  Generally, the IE report was favorable to the process employed and the outcomes included in the modeling.  Certain recommendations for future ERP processes were provided with a primary focus regarding enhanced modeling of new resource types such as battery storage.

On July 23, 2018, various stakeholders commented on the 120-day update report for the ERP and the CEP. Many community, advocate and developer interests supported the CEP, while certain stakeholders opposed the CEP and the associated early coal plant retirements. The CPUC staff indicated that PSCo’s preferred CEP plan is a valid option, but expressed concerns on the saving assumptions, complexity of modeling and the utilization of production tax credits.

A CPUC decision is anticipated in September 2018.

Public Utility Regulatory Policies Act (PURPA) Enforcement Complaint against CPUC — Sustainable Power Group, LLC (sPower) has proposed to construct over 1,500 MW of solar and wind generation in Colorado and is seeking to require PSCo to contract for these resources under PURPA. In March 2017, sPower filed a complaint for declaratory and injunctive relief in the United States District Court for the District of Colorado (District Court) requesting that the court find a December 2016 CPUC ruling that a qualifying facility must be a successful bidder in a PSCo resource acquisition bidding process violated PURPA and FERC rules. In June 2018, the court denied a motion by the CPUC to dismiss. The case remains pending further action from the court.

30

Open Access Transmission Tariff (OATT) Reform — In May 2018, the FERC denied a request by PSCo to amend its OATT to allow large generating interconnection agreements to be suspended by the generator only due to a force majeure event, rather than allowing suspension for up to 36 months for any reason.  PSCo requested the changes to facilitate more efficient processing of generator interconnection requests.  PSCo has initiated a process to achieve broader generator interconnection queue reform and anticipates requesting additional OATT changes later in 2018.  In April, 2018, the FERC had issued a final rule requiring generator interconnection OATT queue reforms in addition (but generally complimentary) to reforms PSCo already requested. PSCo currently has more than 22,000 MW of new generator projects in its interconnection queue.  The broader interconnection queue reforms are intended to allow generators to proceed to interconnection on a “first ready, first served” basis, similar to the processes already use in MISO.

Boulder, Colorado Municipalization — In 2011, City of Boulder, Colorado (Boulder) voters passed a ballot measure authorizing the formation of an electric municipal utility, subject to certain conditions. Since that time, there have been various legal proceedings in multiple venues with jurisdiction over Boulder’s plan. In 2014, the Boulder City Council passed an ordinance to establish an electric utility. PSCo challenged the formation of this utility as premature and the Colorado Court of Appeals ruled in PSCo’s favor, vacating a lower court decision. In June 2018, the Colorado Supreme Court rejected Boulder’s request to dismiss the case and ruled that the case be remanded for hearing at the Boulder District Court (District Court).

Boulder has filed multiple separation applications with the CPUC, which have been challenged by PSCo and other intervenors. In September 2017, the CPUC issued a written decision, agreeing with several key aspects of PSCo’s position. The CPUC approved the designation of some electrical distribution assets for transfer, subject to Boulder completing certain filings. In July 2018, the CPUC approved Boulder and PSCo’s joint request to extend the time by which these filings would be due until Aug. 24, 2018. Boulder does not have authorization from the CPUC to initiate a condemnation proceeding at this time.

Summary of Recent Federal Regulatory Developments

FERC

The FERC has jurisdiction over rates for electric transmission service in interstate commerce and electricity sold at wholesale, hydro facility licensing, natural gas transportation, asset transactions and mergers, accounting practices and certain other activities of PSCo, including enforcement of North American Electric Reliability Corporation mandatory electric reliability standards. State and local agencies have jurisdiction over many of PSCo’s activities, including regulation of retail rates and environmental matters. See additional discussion in the summary of recent federal regulatory developments and public utility regulation sections of the PSCo Annual Report on Form 10-K for the year ended Dec. 31, 2017 and Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018. In addition to the matters discussed below, see Note 5 to the consolidated financial statements for a discussion of other regulatory matters.

Xcel Energy, which includes PSCo, attempts to mitigate the risk of regulatory penalties through formal training on prohibited practices and a compliance function that reviews interaction with the markets under FERC and Commodity Futures Trading Commission jurisdictions. Public campaigns are conducted to raise awareness of the public safety issues of interacting with our electric systems. While programs to comply with regulatory requirements are in place, there is no guarantee the compliance programs or other measures will be sufficient to ensure against violations.

Item 4 — CONTROLS AND PROCEDURES

Disclosure Controls and Procedures

PSCo maintains a set of disclosure controls and procedures designed to ensure that information required to be disclosed in reports that it files or submits under the Securities Exchange Act of 1934 is recorded, processed, summarized, and reported within the time periods specified in SEC rules and forms. In addition, the disclosure controls and procedures ensure that information required to be disclosed is accumulated and communicated to management, including the chief executive officer (CEO) and chief financial officer (CFO), allowing timely decisions regarding required disclosure. As of June 30, 2018, based on an evaluation carried out under the supervision and with the participation of PSCo’s management, including the CEO and CFO, of the effectiveness of its disclosure controls and the procedures, the CEO and CFO have concluded that PSCo’s disclosure controls and procedures were effective.

Internal Control Over Financial Reporting

No changes in PSCo’s internal control over financial reporting occurred during the most recent fiscal quarter that materially affected, or are reasonably likely to materially affect, PSCo’s internal control over financial reporting.

31

Part II — OTHER INFORMATION

Item 1 — LEGAL PROCEEDINGS

PSCo is involved in various litigation matters that are being defended and handled in the ordinary course of business. The assessment of whether a loss is probable or is a reasonable possibility, and whether the loss or a range of loss is estimable, often involves a series of complex judgments about future events. Management maintains accruals for such losses that are probable of being incurred and subject to reasonable estimation. Management is sometimes unable to estimate an amount or range of a reasonably possible loss in certain situations, including but not limited to when (1) the damages sought are indeterminate, (2) the proceedings are in the early stages, or (3) the matters involve novel or unsettled legal theories. In such cases, there is considerable uncertainty regarding the timing or ultimate resolution of such matters, including a possible eventual loss.

Additional Information

See Note 6 to the consolidated financial statements for further discussion of legal claims and environmental proceedings. See Part I Item 2 and Note 5 to the consolidated financial statements for a discussion of proceedings involving utility rates and other regulatory matters.

Item 1A — RISK FACTORS

PSCo’s risk factors are documented in Item 1A of Part I of its Annual Report on Form 10-K for the year ended Dec. 31, 2017, which is incorporated herein by reference. There have been no material changes from the risk factors previously disclosed in the Form 10-K.

Item 6 — EXHIBITS

* Indicates incorporation by reference

3.01*	Amended and Restated Articles of Incorporation dated July 15, 1998 (Exhibit 3.01 to Form 10-Q for the quarter ended Sept. 30, 2017 (file no. 001-03280)).
3.02*	By-Laws of PSCo as Amended and Restated on Sept. 26, 2013 (Exhibit 3.02 to Form 10-Q/A for the quarter ended Sept. 30, 2013 (file no. 001-03280)).
4.01*	Supplemental Indenture dated as of June 1, 2018, between Public Service Company of Colorado and U.S. Bank National Association, as successor Trustee, creating $350 million principal amount of 3.70 percent First Mortgage Bonds, Series No. 31 due 2028, and $350 million principal amount of 4.10 percent First Mortgage Bonds, Series No. 32 due 2048 (Exhibit 4.01 to Form 8-K filed June 21, 2018 (file no. 001-03280)).
10.01*	Seventh Amendment dated May 7, 2018 to the Xcel Energy Senior Executive Severance and Change-in-Control Policy (Exhibit 10.01 to Form 10-Q of Xcel Energy for the quarter ended June 30, 2018 (file no. 001-03034)).
31.01	Principal Executive Officer’s certifications pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 302 of the Sarbanes-Oxley Act of 2002.
31.02	Principal Financial Officer’s certifications pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 302 of the Sarbanes-Oxley Act of 2002.
32.01	Certification pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002.
99.01	Statement pursuant to Private Securities Litigation Reform Act of 1995.
101	The following materials from PSCo’s Quarterly Report on Form 10-Q for the quarter ended June 30, 2018 are formatted in XBRL (eXtensible Business Reporting Language):  (i) the Consolidated Statements of Income, (ii) the Consolidated Statements of Comprehensive Income (iii) the Consolidated Statements of Cash Flows, (iv) the Consolidated Balance Sheets, (v) Notes to Consolidated Financial Statements, and (vi) document and entity information.

32

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

		Public Service Company of Colorado
July 27, 2018	By:	/s/ JEFFREY S. SAVAGE
		Jeffrey S. Savage
		Senior Vice President, Controller
		(Principal Accounting Officer)
		/s/ ROBERT C. FRENZEL
		Robert C. Frenzel
		Executive Vice President, Chief Financial Officer and Director
		(Principal Financial Officer)

33

EX-31.01

Exhibit 31.01

CERTIFICATION

I, Ben Fowke, certify that:

1. I have reviewed this report on Form 10-Q of Public Service Company of Colorado;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations and cash flows of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant and have:

a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

d) Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the registrant’s most recent fiscal quarter (the registrant’s fourth fiscal quarter in the case of an annual report) that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrant’s other certifying officer and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the registrant’s auditors and the audit committee of the registrant’s board of directors (or persons performing the equivalent functions):

a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize and report financial information; and

b) Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: July 27, 2018

/s/ BEN FOWKE

Ben Fowke

Chairman, Chief Executive Officer and Director

(Principal Executive Officer)

1

EX-31.02

Exhibit 31.02

CERTIFICATION

I, Robert C. Frenzel, certify that:

1. I have reviewed this report on Form 10-Q of Public Service Company of Colorado;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations and cash flows of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant and have:

a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

d) Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the registrant’s most recent fiscal quarter (the registrant’s fourth fiscal quarter in the case of an annual report) that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrant’s other certifying officer and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the registrant’s auditors and the audit committee of the registrant’s board of directors (or persons performing the equivalent functions):

a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize and report financial information; and

b) Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: July 27, 2018

/s/ ROBERT C. FRENZEL

Robert C. Frenzel

Executive Vice President, Chief Financial Officer and Director

(Principal Financial Officer)

2

EX-32.01

Exhibit 32.01

OFFICER CERTIFICATION

CERTIFICATION PURSUANT TO

18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Public Service Company of Colorado (PSCo) on Form 10-Q for the quarter ended June 30, 2018, as filed with the SEC on the date hereof (Form 10-Q), each of the undersigned officers of PSCo certifies, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that, to such officer’s knowledge:

(1) The Form 10-Q fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2) The information contained in the Form 10-Q fairly presents, in all material respects, the financial condition and results of operations of PSCo as of the dates and for the periods expressed in the Form 10-Q.

Date: July 27, 2018

/s/ BEN FOWKE

Ben Fowke

Chairman, Chief Executive Officer and Director

(Principal Executive Officer)

/s/ ROBERT C. FRENZEL

Robert C. Frenzel

Executive Vice President, Chief Financial Officer and Director

(Principal Financial Officer)

The foregoing certification is being furnished solely pursuant to 18 U.S.C. Section 1350 and is not being filed as part of the Report or as a separate disclosure document.

A signed original of this written statement required by Section 906, or other document authenticating, acknowledging or otherwise adopting the signature that appears in typed form within the electronic version of this written statement required by Section 906, has been provided to PSCo and will be retained by PSCo and furnished to the SEC or its staff upon request.

1

EX-99.01

Exhibit 99.01

Public Service Company of Colorado Cautionary Factors

The Private Securities Litigation Reform Act provides a “safe harbor” for forward-looking statements to encourage such disclosures without the threat of litigation, providing those statements are identified as forward-looking and are accompanied by meaningful, cautionary statements identifying important factors that could cause the actual results to differ materially from those projected in the statement.  Forward-looking statements are made in written documents and oral presentations of PSCo.  These statements are based on management’s beliefs as well as assumptions and information currently available to management. Such forward-looking statements are intended to be identified in this document by the words “anticipate,” “believe,” “estimate,” “expect,” “intend,” “may,” “objective,” “outlook,” “plan,” “project,” “possible,” “potential,” “should” and similar expressions. In addition to any assumptions and other factors referred to specifically in connection with such forward-looking statements, factors that could cause PSCo’s actual results to differ materially from those contemplated in any forward-looking statements include, among others, the following:

• Economic conditions, including inflation rates, monetary fluctuations and their impact on capital expenditures;

• The risk of a significant slowdown in growth or decline in the U.S. economy, the risk of delay in growth recovery in the U.S. economy or the risk of increased cost for insurance premiums, security and other items as a consequence of past or future terrorist attacks;

• Trade, monetary, fiscal, taxation and environmental policies of governments, agencies and similar organizations in geographic areas where PSCo has a financial interest;

• Customer business conditions, including demand for their products or services and supply of labor and materials used in creating their products and services;

• Financial or regulatory accounting principles or policies imposed by the FASB, the SEC, the Federal Energy Regulatory Commission and similar entities with regulatory oversight;

• Availability of cost or capital such as changes in: interest rates; market perceptions of the utility industry, PSCo, Xcel Energy Inc. or any of its other subsidiaries; or security ratings;

• Factors affecting utility and nonutility operations such as unusual weather conditions; catastrophic weather-related damage; unscheduled generation outages, maintenance or repairs; unanticipated changes to fossil fuel or natural gas supply costs or availability due to higher demand, shortages, transportation problems or other developments; environmental incidents; cyber incidents; or electric transmission or natural gas pipeline constraints;

• Employee workforce factors, including loss or retirement of key executives, collective-bargaining agreements with union employees, or work stoppages;

• Increased competition in the utility industry or additional competition in the markets served by PSCo, Xcel Energy Inc. or any of its other subsidiaries;

• State, federal and foreign legislative and regulatory initiatives that affect cost and investment recovery, have an impact on rate structures and affect the speed and degree to which competition enters the electric and natural gas markets; industry restructuring initiatives; transmission system operation and/or administration initiatives; recovery of investments made under traditional regulation; nature of competitors entering the industry; retail wheeling; a new pricing structure; and former customers entering the generation market;

• Environmental laws and regulations, including legislation and regulations relating to climate change, and the associated cost of   compliance;

• Rate-setting policies or procedures of regulatory entities, including environmental externalities, which are values established by regulators assigning environmental costs to each method of electricity generation when evaluating generation resource options;

• Social attitudes regarding the utility and power industries;

• Cost and other effects of legal and administrative proceedings, settlements, investigations and claims;

• Technological developments that result in competitive disadvantages and create the potential for impairment of existing assets;

• Risks associated with implementations of new technologies; and

• Other business or investment considerations that may be disclosed from time to time in PSCo’s SEC filings, including “Risk Factors” in Item 1A of PSCo’s Form 10-K for the year ended Dec. 31, 2017, or in other publicly disseminated written documents.

PSCo undertakes no obligation to publicly update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.  The foregoing review of factors should not be construed as exhaustive.

XML

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 27, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	PUBLIC SERVICE CO OF COLORADO
Entity Central Index Key	0000081018
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		100

XML

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues
Electric	$ 716,195	$ 729,920	$ 1,414,469	$ 1,441,308
Natural gas	186,661	192,777	550,647	548,913
Steam and other	9,010	8,219	20,048	21,229
Total operating revenues	911,866	930,916	1,985,164	2,011,450
Operating expenses
Electric fuel and purchased power	271,891	279,522	553,061	568,349
Cost of natural gas sold and transported	58,559	70,258	249,824	266,660
Cost of sales — steam and other	3,664	3,507	7,540	7,893
Operating and maintenance expenses	188,991	187,394	372,066	372,482
Demand side management expenses	33,202	29,928	65,954	58,032
Depreciation and amortization	116,553	117,513	238,160	232,507
Taxes (other than income taxes)	49,743	49,470	102,400	99,268
Total operating expenses	722,603	737,592	1,589,005	1,605,191
Operating income	189,263	193,324	396,159	406,259
Other income, net	799	1,319	1,030	4,523
Allowance for funds used during construction — equity	13,555	6,341	24,499	10,949
Interest charges and financing costs
Interest charges — includes other financing costs of $1,601 and $1,543, $3,173, and $3,064 respectively	51,221	46,424	101,142	92,306
Allowance for funds used during construction — debt	(5,205)	(2,438)	(9,786)	(4,344)
Total interest charges and financing costs	46,016	43,986	91,356	87,962
Income before income taxes	157,601	156,998	330,332	333,769
Income taxes	35,305	56,411	74,314	121,636
Net income	$ 122,296	$ 100,587	$ 256,018	$ 212,133

XML

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest charges and financing costs
Other financing costs	$ 1,601	$ 1,543	$ 3,173	$ 3,064

XML

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive income:
Net income	$ 122,296	$ 100,587	$ 256,018	$ 212,133
Amortization of losses included in net periodic benefit cost, net of tax of $0, $1, $0, and $2, respectively	2	1	4	2
Derivative instruments:
Reclassification of losses to net income, net of tax of $99, $153, $197 and $305, respectively	303	250	603	496
Other comprehensive income	305	251	607	498
Comprehensive income	$ 122,601	$ 100,838	$ 256,625	$ 212,631

XML

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative instruments:
Reclassification of losses (gains) to net income, tax	$ 99	$ 153	$ 197	$ 305
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax	$ 0	$ 1	$ 0	$ 2

XML

CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net income	$ 256,018	$ 212,133
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	240,577	234,143
Demand side management program amortization	0	672
Deferred income taxes	25,579	126,252
Amortization of investment tax credits	(1,399)	(1,401)
Allowance for equity funds used during construction	(24,499)	(10,949)
Net realized and unrealized hedging and derivative transactions	(1,917)	1,951
Other Operating Activities, Cash Flow Statement	1	0
Changes in operating assets and liabilities:
Accounts receivable	21,991	24,042
Accrued unbilled revenues	66,497	81,649
Inventories	44,769	38,452
Prepayments and other	1,522	(4,837)
Accounts payable	(22,097)	(51,894)
Net regulatory assets and liabilities	30,405	(2,499)
Other current liabilities	(119,132)	(67,418)
Pension and other employee benefit obligations	(27,457)	(16,543)
Change in other noncurrent assets	3,880	(717)
Change in other noncurrent liabilities	(14,546)	(228)
Net cash provided by operating activities	480,192	562,808
Investing activities
Utility capital/construction expenditures	(825,463)	(609,369)
Allowance for equity funds used during construction	24,499	10,949
Investments in utility money pool arrangement	(198,000)	(777,000)
Repayments from utility money pool arrangement	56,000	625,000
Payments for (Proceeds from) Other Investing Activities	0	657
Net cash used in investing activities	(942,964)	(751,077)
Financing activities
Repayments of short-term borrowings, net	0	(129,000)
Borrowings under utility money pool arrangement	526,000	40,000
Repayments under utility money pool arrangement	(526,000)	(40,000)
Capital contributions from parent	216,508	82,475
Proceeds from Issuance of Long-term Debt	692,697	394,611
Dividends paid to parent	(171,546)	(161,312)
Other	(118)	(110)
Net cash provided by financing activities	737,541	186,664
Net change in cash and cash equivalents	274,769	(1,605)
Cash and cash equivalents at beginning of period	7,513	5,926
Cash and cash equivalents at end of period	282,282	4,321
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(88,912)	(84,452)
Cash paid for income taxes, net	(96,448)	(12,195)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 131,823	$ 103,774

XML

CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 282,282	$ 7,513
Accounts receivable, net	272,293	294,403
Accounts receivable from affiliates	38,650	14,719
Investments in utility money pool arrangement	162,000	20,000
Accrued unbilled revenues	229,304	295,801
Inventories	169,720	214,489
Regulatory assets	76,419	77,337
Derivative instruments	5,340	3,197
Prepayments and other	39,237	35,720
Total current assets	1,275,245	963,179
Property, plant and equipment, net	14,541,078	14,025,751
Other assets
Regulatory assets	980,811	950,258
Derivative instruments	2,716	1,009
Other	23,259	27,429
Total other assets	1,006,786	978,696
Total assets	16,823,109	15,967,626
Current liabilities
Current portion of long-term debt	705,869	305,577
Accounts payable	433,822	492,829
Accounts payable to affiliates	38,395	58,749
Regulatory liabilities	95,053	66,126
Taxes accrued	108,812	222,517
Accrued interest	49,803	48,552
Dividends payable to parent	100,338	76,195
Derivative instruments	5,428	7,348
Other	92,583	92,333
Total current liabilities	1,630,103	1,370,226
Deferred credits and other liabilities
Deferred income taxes	1,678,207	1,644,476
Deferred investment tax credits	26,459	27,858
Regulatory liabilities	1,926,944	1,933,488
Asset retirement obligations	355,028	347,769
Derivative instruments	2,471	3,468
Customer Advances for Construction	170,659	162,614
Pension and employee benefit obligations	260,012	287,783
Other	49,454	58,923
Total deferred credits and other liabilities	4,469,234	4,466,379
Commitments and contingencies
Capitalization
Long-term debt	4,594,193	4,302,698
Common stock — 100 shares authorized at $0.01 par value; 100 shares outstanding at June 30, 2018 and Dec. 31, 2017, respectively	0	0
Additional paid in capital	4,273,146	4,032,826
Retained earnings	1,882,558	1,822,229
Accumulated other comprehensive loss	(26,125)	(26,732)
Total common stockholder’s equity	6,129,579	5,828,323
Total liabilities and equity	$ 16,823,109	$ 15,967,626

XML

CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Capitalization, Long-term Debt and Equity [Abstract]
Common stock, shares authorized (in shares)	100	100
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares outstanding (in shares)	100	100

XML

Management's Opinion

6 Months Ended

Jun. 30, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Management's Opinion

In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary to present fairly, in accordance with accounting principles generally accepted in the United States of America (GAAP), the financial position of PSCo and its subsidiaries as of June 30, 2018 and Dec. 31, 2017; the results of its operations, including the components of net income and comprehensive income, for the three and six months ended June 30, 2018 and 2017; and its cash flows for the six months endedJune 30, 2018 and 2017. All adjustments are of a normal, recurring nature, except as otherwise disclosed. Management has also evaluated the impact of events occurring after June 30, 2018 up to the date of issuance of these consolidated financial statements. These statements contain all necessary adjustments and disclosures resulting from that evaluation. The Dec. 31, 2017 balance sheet information has been derived from the audited 2017 consolidated financial statements included in the PSCo Annual Report on Form 10-K for the year ended Dec. 31, 2017. These notes to the consolidated financial statements have been prepared pursuant to the rules and regulations of the SEC for Quarterly Reports on Form 10-Q. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP on an annual basis have been condensed or omitted pursuant to such rules and regulations. For further information, refer to the consolidated financial statements and notes thereto, included in the PSCo Annual Report on Form 10-K for the year ended Dec. 31, 2017, filed with the SEC on Feb. 23, 2018. Due to the seasonality of PSCo’s electric and natural gas sales, interim results are not necessarily an appropriate base from which to project annual results.

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Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

The significant accounting policies set forth in Note 1 to the consolidated financial statements in the PSCo Annual Report on Form 10-K for the year ended Dec. 31, 2017, appropriately represent, in all material respects, the current status of accounting policies and are incorporated herein by reference.

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Accounting Pronouncements

6 Months Ended

Jun. 30, 2018

New Accounting Pronouncements and Changes in Accounting Principles [Abstract]

Accounting Pronouncements

Accounting Pronouncements

Recently Issued

Leases —In February 2016, the Financial Accounting Standards Board (FASB) issued Leases, Topic 842 (Accounting Standards Update (ASU) No. 2016-02), which for lessees requires balance sheet recognition of right-of-use assets and lease liabilities for most leases. This guidance will be effective for interim and annual reporting periods beginning after Dec. 15, 2018. PSCo has not yet fully determined the impacts of implementation. However, adoption is expected to occur on Jan. 1, 2019 utilizing the practical expedients provided by the standard and proposed in Targeted Improvements, Topic 842 (Proposed ASU 2018-200). On Jan. 1, 2019 agreements considered leases for the use of office space, equipment and natural gas storage assets, as well as certain purchased power agreements (PPAs) for fossil-fueled generating facilities are expected to be recognized on the consolidated balance sheet.

Recently Adopted

Revenue Recognition— In May 2014, the FASB issued Revenue from Contracts with Customers, Topic 606 (ASU No. 2014-09), which provides a new framework for the recognition of revenue. PSCo implemented the guidance on a modified retrospective basis on Jan. 1, 2018. Results for reporting periods beginning after Dec. 31, 2017 are presented in accordance with Topic 606, while prior period results have not been adjusted and continue to be reported in accordance with prior accounting guidance. Other than increased disclosures regarding revenues related to contracts with customers, the implementation did not have a significant impact on PSCo’s consolidated financial statements. For related disclosures, see Note 13 to the consolidated financial statements.

Classification and Measurement of Financial Instruments — In January 2016, the FASB issued Recognition and Measurement of Financial Assets and Financial Liabilities, Subtopic 825-10 (ASU No. 2016-01), which eliminated the available-for-sale classification for marketable equity securities and also replaced the cost method of accounting for non-marketable equity securities with a model for recognizing impairments and observable price changes. Under the new standard, other than when the consolidation or equity method of accounting is utilized, changes in the fair value of equity securities are recognized in earnings. PSCo implemented the guidance on Jan. 1, 2018 and the implementation did not have a material impact on its consolidated financial statements.

Presentation of Net Periodic Benefit Cost —In March 2017, the FASB issued Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, Topic 715 (ASU No. 2017-07), which establishes that only the service cost element of pension cost may be presented as a component of operating income in the income statement. Also under the guidance, only the service cost component of pension cost is eligible for capitalization. As a result of the application of accounting principles for rate regulated entities, a similar amount of pension cost, including non-service components, will be recognized consistent with the historical ratemaking treatment, and the impacts of adoption will be limited to changes in classification of non-service costs in the consolidated statement of income. PSCo implemented the new guidance on Jan. 1, 2018, and as a result, $1.0 million of pension costs were retrospectively reclassified from operating and maintenance expenses to other income, net on the consolidated income statement for the six months ended June 30, 2017. Under a practical expedient permitted by the standard, PSCo used benefit cost amounts disclosed for prior periods as the basis for retrospective application.

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Selected Balance Sheet Data

6 Months Ended

Jun. 30, 2018

Balance Sheet Related Disclosures [Abstract]

Selected Balance Sheet Data

Selected Balance Sheet Data

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Accounts receivable, net
Accounts receivable	$	290,832		$	314,009
Less allowance for bad debts	(18,539		)	(19,606		)
	$	272,293		$	294,403

(Thousands of Dollars)	June 30, 2018		Dec. 31, 2017
Inventories
Materials and supplies	$	70,940	$	68,940
Fuel	62,532		73,893
Natural gas	36,248		71,656
	$	169,720	$	214,489

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Property, plant and equipment, net
Electric plant	$	12,780,963		$	12,627,592
Natural gas plant	4,211,977			4,102,075
Common and other property	1,040,638			1,022,333
Plant to be retired (a)	10,306			10,949
Construction work in progress	1,444,140			1,014,338
Total property, plant and equipment	19,488,024			18,777,287
Less accumulated depreciation	(4,946,946		)	(4,751,536		)
	$	14,541,078		$	14,025,751

^(a)  In the third quarter of 2017, PSCo early retired Valmont Unit 5 and converted Cherokee Unit 4 from a coal-fueled generating facility to natural gas. PSCo also expects Craig Unit 1 to be early retired in approximately 2025. Amounts are presented net of accumulated depreciation.

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Income Taxes

6 Months Ended

Jun. 30, 2018

Income Tax Disclosure [Abstract]

Income Taxes

Income Taxes

Except to the extent noted below, Note 7 to the consolidated financial statements included in PSCo’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 appropriately represents, in all material respects, the current status of other income tax matters, and are incorporated herein by reference.

Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences:

	Six Months Ended June 30
	2018		2017
Federal statutory rate	21.0	%	35.0	%
State tax, net of federal tax effect	3.7		3.0
Increases (decreases) in tax from:
Regulatory differences - ARAM (a)	(3.2	)	(0.1	)
Regulatory differences - ARAM deferral (b)	3.0		—
Regulatory differences - other utility plant items	(1.3	)	(0.9	)
Tax credits, net of federal income tax expense	(1.0	)	(0.7	)
Other, net	0.3		0.1
Effective income tax rate	22.5	%	36.4	%

^(a)   The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.

^(b) As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the Tax Cuts and Jobs Act

(TCJA)), the ARAM deferral may decrease during the year, which would result in a reduction to tax expense with a corresponding reduction to revenue.

Federal Audits— PSCO is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows:

Tax Year(s)	Expiration
2009 - 2011	December 2018
2012 - 2014	October 2019
2015	September 2019
2016	September 2020

In 2012, the Internal Revenue Service (IRS) commenced an examination of tax years 2010 and 2011, including the 2009 carryback claim. The IRS proposed an adjustment to the federal tax loss carryback claims and in 2015, the IRS forwarded the issue to the Office of Appeals (Appeals). In 2017 Xcel Energy and Appeals reached an agreement and the benefit related to the agreed upon portions was recognized. PSCo did not accrue any income tax benefit related to this adjustment. In the second quarter of 2018, the Joint Committee on Taxation completed its review and took no exception to the agreement. As a result, the remaining unrecognized tax benefit was released and recorded as a payable to the IRS.

In the third quarter of 2015, the IRS commenced an examination of tax years 2012 and 2013. In the third quarter of 2017, the IRS concluded the audit of tax years 2012 and 2013 and proposed an adjustment that would impact Xcel Energy’s net operating loss (NOL) and effective tax rate (ETR). After evaluating the proposed adjustment Xcel Energy filed a protest with the IRS. As of June 30, 2018, the case has been forwarded to Appeals and Xcel Energy has recognized its best estimate of income tax expense that will result from a final resolution of this issue; however, the outcome and timing of a resolution is unknown.

State Audits — PSCo is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of June 30, 2018, PSCo’s earliest open tax year that is subject to examination by state taxing authorities under applicable statutes of limitations is 2009. There are currently no state income tax audits in progress.

Unrecognized Benefits— The unrecognized tax benefit balance includes permanent tax positions, which if recognized would affect the annual ETR. In addition, the unrecognized tax benefit balance includes temporary tax positions for which the ultimate deductibility is highly certain but for which there is uncertainty about the timing of such deductibility. A change in the period of deductibility would not affect the ETR but would accelerate the payment of cash to the taxing authority to an earlier period.

A reconciliation of the amount of unrecognized tax benefit is as follows:

(Millions of Dollars)	June 30, 2018		Dec. 31, 2017
Unrecognized tax benefit — Permanent tax positions	$	4.4	$	4.0
Unrecognized tax benefit — Temporary tax positions	5.0		6.1
Total unrecognized tax benefit	$	9.4	$	10.1

The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows:

(Millions of Dollars)	June 30, 2018			Dec. 31, 2017
NOL and tax credit carryforwards	$	(4.7	)	$	(4.0	)

It is reasonably possible that PSCo’s amount of unrecognized tax benefits could significantly change in the next 12 months as the IRS Appeals progresses and the IRS and state audits resume. As the IRS Appeals progresses and the IRS audit resumes, it is reasonably possible that the amount of unrecognized tax benefit could decrease up to approximately $8 million.

The payable for interest related to unrecognized tax benefits is partially offset by the interest benefit associated with NOL and tax credit carryforwards. The payables for interest related to unrecognized tax benefits at June 30, 2018, and Dec. 31, 2017 were not material. No amounts were accrued for penalties related to unrecognized tax benefits as of June 30, 2018 or Dec. 31, 2017.

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Rate Matters

6 Months Ended

Jun. 30, 2018

Public Utilities, General Disclosures [Abstract]

Rate Matters

Rate Matters

Except to the extent noted below, the circumstances set forth in Note 11 to the consolidated financial statements included in PSCo’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Note 5 to the consolidated financial statements to PSCo’s Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018, appropriately represent, in all material respects, the current status of other rate matters, and are incorporated herein by reference.

Tax Reform — Regulatory Proceedings

The specific impacts of the TCJA on customer rates are subject to regulatory approval. Each of the states in Xcel Energy’s service areas, including Colorado, have opened dockets to address the impacts of the TCJA.

Colorado Natural Gas — In February 2018, the administrative law judge (ALJ) approved PSCo and the Colorado Public Utilities Commission (CPUC) Staff’s TCJA settlement agreement which includes a $20 million reduction to provisional rates effective March 1, 2018. A final true-up would provide customers the full net benefit of the TCJA retroactive to January 2018.

Colorado Electric— In April 2018, PSCo, the CPUC Staff and the Office of Consumer Counsel (OCC) filed a TCJA settlement agreement that recommended a customer refund of $42 million in 2018, with the remainder of $59 million be used to accelerate the amortization of an existing prepaid pension asset.  In June 2018, the CPUC approved the customer refund of $42 million, effective June 1, 2018. The CPUC set the decision regarding the remainder of the $59 million for hearing before an ALJ. Revisions to the TCJA settlement will be addressed in a future electric rate case.

Pending Regulatory Proceedings — CPUC

Colorado 2017 Multi-Year Electric Rate Case — In October 2017, PSCo filed a multi-year request with the CPUC seeking to increase electric rates approximately $245 million over four years. The request was based on forecast test years (FTY), a 10.0 percent return on equity (ROE) and an equity ratio of 55.25 percent. Interim rates, subject to refund and interest, were to be effective on June 1, 2018.

Revenue Request (Millions of Dollars)	2018		2019		2020		2021		Total
Revenue request	$	74	$	75	$	60	$	36	$	245
Clean Air Clean Jobs Act (CACJA) rider conversion to base rates	90		—		—		—		90
Transmission Cost Adjustment (TCA) rider conversion to base rates	43		—		—		—		43
Total	$	207	$	75	$	60	$	36	$	378
Expected year-end rate base (billions of dollars)	$	6.8	$	7.1	$	7.3	$	7.4

In March 2018, PSCo, CPUC Staff and OCC reached a settlement and filed a motion with the CPUC requesting changes to the procedural schedule and scope of the electric case, which included delaying the implementation of provisional rates from June 2018 to January 2019 and requiring PSCo to file updated test year information for 2019 through 2021 which included the impacts of TCJA. In April 2018, the CPUC denied the motion on procedural grounds and dismissed the electric rate case.

Colorado 2017 Multi-Year Natural Gas Rate Case — In June 2017, PSCo filed a multi-year request with the CPUC seeking to increase retail natural gas rates approximately $139 million over three years. The request, detailed below, was based on FTYs, a 10.0 percent ROE and an equity ratio of 55.25 percent.

Revenue Request (Millions of Dollars)	2018		2019		2020		Total
Revenue request	$	63	$	33	$	43	$	139
Pipeline System Integrity Adjustment (PSIA) rider conversion to base rates (a)	—		94		—		94
Total	$	63	$	127	$	43	$	233
Expected year-end rate base (billions of dollars) (b)	$	1.5	$	2.3	$	2.4

^(a)   The roll-in of PSIA rider revenue into base rates will not have an impact on customer bills or revenue as these costs are already being recovered through the rider. The recovery of incremental PSIA related investments in 2019 and 2020 are included in the base rate request.

^(b)   The additional rate base in 2019 predominantly reflects the roll-in of capital associated with the PSIA rider.

In February 2018, the administrative law judge (ALJ) approved a TCJA settlement agreement between PSCo and the CPUC Staff, which reduced provisional rates by $20 million, based on a preliminary TCJA estimate of $29 million. The settlement remains subject to CPUC approval. The impact of the TCJA will be trued-up later in 2018. Annualized provisional rates of approximately $43 million were effective March 1, 2018.

In May 2018, the ALJ issued an interim recommended decision which would result in a 2018 overall rate increase of approximately $46 million, prior to the impact of the TCJA. The estimated rate increase reflects a 2016 HTY with a 13-month average rate base of $1.6 billion, a ROE of 9.35 percent and an equity ratio of 54.2 percent.

On July 12, 2018, the CPUC deliberated and approved several of the ALJ’s recommendations including application of a 2016 HTY, with a 13-month average rate base, and an ROE of 9.35 percent.  The CPUC adjusted the equity ratio to 54.6 percent and provided no return on the prepaid pension and retiree medical asset.  With these adjustments the total rate increase, prior to TCJA impacts, would be $47 million.

The estimated impact of the CPUC’s decision is presented below:

(Millions of Dollars)	Estimated Impact of the CPUC’s Decision
Filed 2018 revenue request based on a FTY	$	63
Impact of the change in test year	5
PSCo’s deficiency based on a 2016 HTY - year-end rate base	68
Adjustments:
ROE at 9.35 percent	(9		)
Equity ratio of 54.6 percent	(2		)
Change in amortization period for certain regulatory assets, including a debt return	(6		)
Loss of return on prepaid pension and retiree medical	(4		)
Change from 2016 year-end to average rate base	(5		)
Other, net	5
Total adjustments	(21		)
Total rate increase, prior to the TCJA impacts	$	47

The CPUC is expected to issue its order on the natural gas rate case in the third quarter of 2018. The CPUC is expected to issue a final decision with the impacts of the TCJA, later in 2018.

Provisional rates, subject to refund, were implemented on Jan. 1, 2018. A current liability which represents PSCo’s best estimate of a refund obligation associated with provisional rates was recorded as of June 30, 2018.

PSIA Rider

In June 2018, PSCo filed for an extension to the PSIA rider through 2020. PSCo requested an expedited decision by Nov. 15, 2018. PSCo also requested authorization to roll-in recovery of costs in the current PSIA rider into base rates effective Jan. 1, 2019, if the CPUC rejects the proposed PSIA extension or fails to rule on the request by the end of 2018.

Additionally, PSCo reduced PSIA revenues by approximately $8 million for 2018 for the impact of the TCJA, effective May 1, 2018. PSIA revenues are subject to the CPUC approved PSIA rider true-up process.

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Commitments and Contingencies

6 Months Ended

Jun. 30, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Commitments and Contingencies

Except to the extent noted below and in Note 5 to the consolidated financial statements, the circumstances set forth in Notes 11 and 12 to the consolidated financial statements included in PSCo’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Notes 5 and 6 to PSCo’s Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018, appropriately represent, in all material respects, the current status of commitments and contingent liabilities and are incorporated herein by reference. The following include commitments, contingencies and unresolved contingencies that are material to PSCo’s financial position.

PPAs

PSCo purchases power from independent power producing entities that own natural gas fueled power plants for which PSCo is required to reimburse natural gas fuel costs, or to participate in tolling arrangements under which PSCo procures the natural gas required to produce the energy that it purchases. These specific PPAs create a variable interest in the associated independent power producing entity.

PSCo had approximately 1,571 Megawatts (MW) of capacity under long-term PPAs as of June 30, 2018 and Dec. 31, 2017, with entities that have been determined to be variable interest entities. PSCo has concluded that these entities are not required to be consolidated in its consolidated financial statements because it does not have the power to direct the activities that most significantly impact the entities’ economic performance. These agreements have various expiration dates through 2032.

Environmental Contingencies

Manufactured Gas Plant (MGP), Landfill or Disposal Sites — PSCo is currently involved in investigating and/or remediating several MGP, landfill or other disposal sites. PSCo has identified four sites where contamination is present and where investigation and/or remediation activities are currently underway. Other parties may have responsibility for some portion of the investigation and/or remediation activities. PSCo anticipates that these investigation or remediation activities will continue through at least 2018. PSCo accrued $2 million as of June 30, 2018 and an immaterial amount as of Dec. 31, 2017, for these sites. There may be insurance recovery and/or recovery from other potentially responsible parties that will offset any costs incurred. PSCo anticipates that any amounts spent will be fully recovered from customers.

Environmental Requirements

Air

Revisions to the National Ambient Air Quality Standard (NAAQS) for Ozone - In 2015, the EPA revised the NAAQS for ozone by lowering the eight-hour standard from 75 parts per billion (ppb) to 70 ppb. PSCo meets the 2015 ozone standard in all areas where its generating units operate, except for the Denver Metropolitan Area. PSCo’s retirement of its coal fired plants in the Denver non-attainment area helped Colorado’s plan to mitigate non-attainment. In June 2018, the EPA designated the parts of the Denver Metropolitan Area that currently do not attain the 2008 ozone standards as also not attaining the more stringent 2015 ozone standard. Colorado will continue to consider further reductions that are available in the non-attainment area as it develops plans to meet the ozone standards. The gas plants that operate in PSCo’s non-attainment area may be required to improve or add controls, implement further work practices and/or implement enhanced emissions monitoring as part of future Colorado state plans.

Legal Contingencies

PSCo is involved in various litigation matters that are being defended and handled in the ordinary course of business. The assessment of whether a loss is probable or is a reasonable possibility, and whether the loss or a range of loss is estimable, often involves a series of complex judgments about future events. Management maintains accruals for such losses that are probable of being incurred and subject to reasonable estimation. Management is sometimes unable to estimate an amount or range of a reasonably possible loss in certain situations, including but not limited to when (1) the damages sought are indeterminate, (2) the proceedings are in the early stages, or (3) the matters involve novel or unsettled legal theories. In such cases, there is considerable uncertainty regarding the timing or ultimate resolution of such matters, including a possible eventual loss. For current proceedings not specifically reported herein, management does not anticipate that the ultimate liabilities, if any, arising from such current proceedings would have a material effect on PSCo’s financial statements. Unless otherwise required by GAAP, legal fees are expensed as incurred.

Employment, Tort and Commercial Litigation

Line Extension Disputes — In December 2015, Development Recovery Company (DRC) filed a lawsuit in the Denver District Court, stating PSCo failed to award proper allowances and refunds for line extensions to new developments pursuant to the terms of electric and gas service agreements entered into by PSCo and various developers. The dispute involved claims by over fifty developers. In February 2018, the Colorado Supreme Court denied DRC’s petition to appeal the Denver District Court’s dismissal of the lawsuit, effectively terminating this litigation. However, in January 2018, DRC filed a new lawsuit in Boulder County District Court, asserting a single claim that PSCo was required to file its line extension agreements with the CPUC but failed to do so. This claim is substantially similar to the arguments previously raised by DRC. PSCo filed a motion to dismiss this claim, which was granted in May 2018. DRC subsequently filed an appeal to the Colorado Court of Appeals. It is uncertain when a decision will be rendered regarding this appeal.

PSCo has concluded that a loss is remote with respect to this matter as the service agreements were developed to implement CPUC approved tariffs and PSCo has complied with the tariff provisions. Also, if a loss were sustained, PSCo believes it would be allowed to recover these costs through traditional regulatory mechanisms. The amount or range in dispute is presently unknown and no accrual has been recorded for this matter.

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Borrowings and Other Financing Instruments

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

Borrowings and Other Financing Instruments

Borrowings and Other Financing Instruments

Short-Term Borrowings

Money Pool — Xcel Energy Inc. and its utility subsidiaries have established a money pool arrangement that allows for short-term investments in and borrowings between the utility subsidiaries. Xcel Energy Inc. may make investments in the utility subsidiaries at market-based interest rates; however, the money pool arrangement does not allow the utility subsidiaries to make investments in Xcel Energy Inc. Money pool borrowings for PSCo were as follows:

(Amounts in Millions, Except Interest Rates)	Three Months Ended June 30, 2018			Year Ended Dec. 31, 2017
Borrowing limit	$	250		$	250
Amount outstanding at period end	—			—
Average amount outstanding	60			—
Maximum amount outstanding	156			20
Weighted average interest rate, computed on a daily basis	1.84		%	0.92		%
Weighted average interest rate at period end	N/A			N/A

Commercial Paper — PSCo meets its short-term liquidity requirements primarily through the issuance of commercial paper and borrowings under its credit facility and the money pool. Commercial paper outstanding for PSCo was as follows:

(Amounts in Millions, Except Interest Rates)	Three Months Ended June 30, 2018			Year Ended Dec. 31, 2017
Borrowing limit	$	700		$	700
Amount outstanding at period end	—			—
Average amount outstanding	84			54
Maximum amount outstanding	257			268
Weighted average interest rate, computed on a daily basis	2.17		%	1.08		%
Weighted average interest rate at period end	N/A			N/A

Letters of Credit— PSCo uses letters of credit, generally with terms of one year, to provide financial guarantees for certain operating obligations. At June 30, 2018 and Dec. 31, 2017, there were $4 million and $3 million, respectively, of letters of credit outstanding under the credit facility. The contract amounts of these letters of credit approximate their fair value and are subject to fees.

Credit Facility — In order to use its commercial paper program to fulfill short-term funding needs, PSCo must have a revolving credit facility in place at least equal to the amount of its commercial paper borrowing limit and cannot issue commercial paper in an aggregate amount exceeding available capacity under this credit facility. The credit facility provides short-term financing in the form of notes payable to banks, letters of credit and back-up support for commercial paper borrowings.

At June 30, 2018, PSCo had the following committed credit facility available (in millions of dollars):

Credit Facility (a)		Drawn(b)		Available
$	700	$	4	$	696

^(a)    This credit facility expires in June 2021.

^(b)    Includes outstanding commercial paper and letters of credit.

All credit facility bank borrowings, outstanding letters of credit and outstanding commercial paper reduce the available capacity under the credit facility. PSCo had no direct advances on the credit facility outstanding at June 30, 2018 and Dec. 31, 2017.

Long-Term Borrowings

PSCo issued $350 million of 3.70 percent first mortgage green bonds due June 15, 2028 and $350 million of 4.10 percent first mortgage green bonds due June 15, 2048.

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Fair Value of Financial Assets and Liabilities

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

Fair Value of Financial Assets and Liabilities

Fair Value Measurements

The accounting guidance for fair value measurements and disclosures provides a single definition of fair value and requires certain disclosures about assets and liabilities measured at fair value. A hierarchical framework for disclosing the observability of the inputs utilized in measuring assets and liabilities at fair value is established by this guidance. The three levels in the hierarchy are as follows:

Level 1 — Quoted prices are available in active markets for identical assets or liabilities as of the reporting date. The types of assets and liabilities included in Level 1 are highly liquid and actively traded instruments with quoted prices.

Level 2 — Pricing inputs are other than quoted prices in active markets, but are either directly or indirectly observable as of the reporting date. The types of assets and liabilities included in Level 2 are typically either comparable to actively traded securities or contracts, or priced with models using highly observable inputs.

Level 3 — Significant inputs to pricing have little or no observability as of the reporting date. The types of assets and liabilities included in Level 3 are those valued with models requiring significant management judgment or estimation.

Specific valuation methods include the following:

Cash equivalents — The fair values of cash equivalents are generally based on cost plus accrued interest; money market funds are measured using quoted net asset value.

Interest rate derivatives— The fair values of interest rate derivatives are based on broker quotes that utilize current market interest rate forecasts.

Commodity derivatives— The methods used to measure the fair value of commodity derivative forwards and options utilize forward prices and volatilities, as well as pricing adjustments for specific delivery locations, and are generally assigned a Level 2 classification. When contractual settlements extend to periods beyond those readily observable on active exchanges or quoted by brokers, the significance of the use of less observable forecasts of long-term forward prices and volatilities on a valuation is evaluated, and may result in Level 3 classification.

Derivative Instruments Fair Value Measurements

PSCo enters into derivative instruments, including forward contracts, futures, swaps and options, for trading purposes and to manage risk in connection with changes in interest rates, utility commodity prices and vehicle fuel prices.

Interest Rate Derivatives — PSCo enters into various instruments that effectively fix the interest payments on certain floating rate debt obligations or effectively fix the yield or price on a specified benchmark interest rate for an anticipated debt issuance for a specific period. These derivative instruments are generally designated as cash flow hedges for accounting purposes.

At June 30, 2018, accumulated other comprehensive losses related to interest rate derivatives included $1.2 million of net losses expected to be reclassified into earnings during the next 12 months as the related hedged interest rate transactions impact earnings, including forecasted amounts for unsettled hedges, as applicable.

Wholesale and Commodity Trading Risk — PSCo conducts various wholesale and commodity trading activities, including the purchase and sale of electric capacity, energy, energy-related instruments and natural gas related instruments, including derivatives. PSCo’s risk management policy allows management to conduct these activities within guidelines and limitations as approved by its risk management committee, which is made up of management personnel not directly involved in the activities governed by this policy.

Commodity Derivatives — PSCo enters into derivative instruments to manage variability of future cash flows from changes in commodity prices in its electric and natural gas operations, as well as for trading purposes. This could include the purchase or sale of energy or energy-related products, natural gas to generate electric energy, natural gas for resale, and vehicle fuel.

PSCo enters into derivative instruments that mitigate commodity price risk on behalf of electric and natural gas customers but may not be designated as qualifying hedging transactions. Changes in the fair value of non-trading commodity derivative instruments are recorded in other comprehensive income or deferred as a regulatory asset or liability. The classification as a regulatory asset or liability is based on commission approved regulatory recovery mechanisms. PSCo had no income related to the ineffectiveness of cash flow hedges for the three and six months ended June 30, 2018 and 2017.

Additionally, PSCo enters into commodity derivative instruments for trading purposes not directly related to commodity price risks associated with serving its electric and natural gas customers. Changes in the fair value of these commodity derivatives are recorded in electric operating revenues, net of amounts credited to customers under margin-sharing mechanisms.

The following table details the gross notional amounts of commodity forwards and options at June 30, 2018 and Dec. 31, 2017:

(Amounts in Thousands)(a)(b)	June 30, 2018	Dec. 31, 2017
Megawatt hours of electricity	20,813	22,260
Million British thermal units of natural gas	18,798	13,410

^(a)  Amounts are not reflective of net positions in the underlying commodities.

^(b)  Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

The following tables detail the impact of derivative activity during the three months ended June 30, 2018 and 2017 on accumulated other comprehensive loss, regulatory assets and liabilities, and income:

	Three Months Ended June 30, 2018
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)		Pre-Tax Gains Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	402	(a)	$	—	$	—
Total	$	—	$	—		$	402		$	—	$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—	$	200	(b)
Natural gas commodity	—		(249		)	—			—		—		(c)
Total	$	—	$	(249	)	$	—		$	—	$	200

	Six Months Ended June 30, 2018
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:						Pre-Tax (Gains) Losses Recognized During the Period in Income
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	800	(a)	$	—		$	—
Total	$	—	$	—		$	800		$	—		$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—		$	724		(b)
Natural gas commodity	—		(420		)	—			2,749		(c)	(1,581		)	(c)
Total	$	—	$	(420	)	$	—		$	2,749		$	(857	)

	Three Months Ended June 30, 2017
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)		Pre-Tax Losses Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	403	(a)	$	—	$	—
Total	$	—	$	—		$	403		$	—	$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—	$	(192	)	(b)
Natural gas commodity	—		(1,621		)	—			—		—			(c)
Total	$	—	$	(1,621	)	$	—		$	—	$	(192	)

	Six Months Ended June 30, 2017
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)			Pre-Tax Gains (Losses) Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	801	(a)	$	—		$	—
Total	$	—	$	—		$	801		$	—		$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—		$	187		(b)
Natural gas commodity	—		(7,008		)	—			282		(c)	(2,990		)	(c)
Total	$	—	$	(7,008	)	$	—		$	282		$	(2,803	)

^(a)  Amounts are recorded to interest charges.

^(b)  Amounts are recorded to interest charges. Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue as appropriate.

^(c)  Certain derivatives are utilized to mitigate natural gas price risk for electric generation and are recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. Amounts for the three and six months endedJune 30, 2018 included no settlement gains or losses and $1.2 million of settlement losses, respectively. Amounts for the three and six months ended June 30, 2017 included no settlement gains or losses and $0.9 million of settlement gains, respectively. The remaining derivative settlement gains and losses for the six months endedJune 30, 2018 and 2017 relate to natural gas operations and are recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

PSCo had no derivative instruments designated as fair value hedges during the three and six months ended June 30, 2018 and 2017. Therefore, no gains or losses from fair value hedges or related hedged transactions were recognized for these periods.

Consideration of Credit Risk and Concentrations — PSCo continuously monitors the creditworthiness of the counterparties to its interest rate derivatives and commodity derivative contracts prior to settlement, and assesses each counterparty’s ability to perform on the transactions set forth in the contracts. Given this assessment, as well as an assessment of the impact of PSCo’s own credit risk when determining the fair value of derivative liabilities, the impact of credit risk was immaterial to the fair value of unsettled commodity derivatives presented in the consolidated balance sheets.

PSCo employs additional credit risk control mechanisms when appropriate, such as letters of credit, parental guarantees, standardized master netting agreements and termination provisions that allow for offsetting of positive and negative exposures. Credit exposure is monitored and, when necessary, the activity with a specific counterparty is limited until credit enhancement is provided.

PSCo’s most significant concentrations of credit risk with particular entities or industries are contracts with counterparties to its wholesale, trading and non-trading commodity activities. At June 30, 2018, four of PSCo’s 10 most significant counterparties for these activities, comprising $27.6 million or 47 percent of this credit exposure, had investment grade credit ratings from Standard & Poor’s, Moody’s or Fitch Ratings. Five of the 10 most significant counterparties, comprising $12.2 million or 21 percent of this credit exposure, were not rated by these external agencies, but based on PSCo’s internal analysis, had credit quality consistent with investment grade. The one remaining significant counterparty, comprising $5.2 million or 9 percent of this credit exposure, had credit quality less than investment grade, based on ratings from external analysis. Nine of these significant counterparties are municipal or cooperative electric entities, or other utilities.

Credit Related Contingent Features—Contract provisions for derivative instruments that PSCo enters into, including those accounted for as normal purchase-normal sale contracts and therefore not reflected on the balance sheet, may require the posting of collateral or settlement of the contracts for various reasons, including if PSCo’s credit ratings are downgraded below its investment grade credit rating by any of the major credit rating agencies or for cross-default contractual provisions that could result in the settlement of such contracts if there was a failure under other financing arrangements related to payment terms or other covenants. At June 30, 2018 and Dec. 31, 2017, there were no derivative instruments in a material liability position with such underlying contract provisions.

Certain derivative instruments are also subject to contract provisions that contain adequate assurance clauses. These provisions allow counterparties to seek performance assurance, including cash collateral, in the event that PSCo’s ability to fulfill its contractual obligations is reasonably expected to be impaired. PSCo had no collateral posted related to adequate assurance clauses in derivative contracts as of June 30, 2018 and Dec. 31, 2017.

Recurring Fair Value Measurements—The following table presents, for each of the fair value hierarchy levels, PSCo’s assets and liabilities measured at fair value on a recurring basis at June 30, 2018:

	June 30, 2018
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative assets
Other derivative instruments:
Commodity trading	$	332	$	12,812	$	118	$	13,262	$	(9,729	)	$	3,533
Natural gas commodity	—		919		—		919		—			919
Total current derivative assets	$	332	$	13,731	$	118	$	14,181	$	(9,729	)	4,452
PPAs (a)												888
Current derivative instruments												$	5,340
Noncurrent derivative assets
Other derivative instruments:
Commodity trading	$	1	$	2,715	$	—	$	2,716	$	—		$	2,716
Total noncurrent derivative assets	$	1	$	2,715	$	—	$	2,716	$	—		2,716
PPAs (a)												—
Noncurrent derivative instruments												$	2,716

	June 30, 2018
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative liabilities
Other derivative instruments:
Commodity trading	$	333	$	12,409	$	—	$	12,742	$	(12,474	)	$	268
Total current derivative liabilities	$	333	$	12,409	$	—	$	12,742	$	(12,474	)	268
PPAs (a)												5,160
Current derivative instruments												$	5,428
Noncurrent derivative liabilities
Other derivative instruments:
Commodity trading	$	—	$	2,382	$	—	$	2,382	$	—		$	2,382
Total noncurrent derivative liabilities	$	—	$	2,382	$	—	$	2,382	$	—		2,382
PPAs (a)												89
Noncurrent derivative instruments												$	2,471

^(a)  During 2006, PSCo qualified these contracts under the normal purchase exception. Based onthis qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

^(b)  PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at June 30, 2018. At June 30, 2018, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $2.7 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

The following table presents, for each of the fair value hierarchy levels, PSCo’s assets and liabilities measured at fair value on a recurring basis at Dec. 31, 2017:

	Dec. 31, 2017
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative assets
Other derivative instruments:
Commodity trading	$	528	$	4,488	$	12	$	5,028	$	(3,554	)	$	1,474
Natural gas commodity	—		18		—		18		(10		)	8
Total current derivative assets	$	528	$	4,506	$	12	$	5,046	$	(3,564	)	1,482
PPAs (a)												1,715
Current derivative instruments												$	3,197
Noncurrent derivative assets
Other derivative instruments:
Commodity trading	$	—	$	1,541	$	—	$	1,541	$	(563	)	$	978
Total noncurrent derivative assets	$	—	$	1,541	$	—	$	1,541	$	(563	)	978
PPAs (a)												31
Noncurrent derivative instruments												$	1,009

	Dec. 31, 2017
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative liabilities
Other derivative instruments:
Commodity trading	$	446	$	4,285	$	6	$	4,737	$	(3,431	)	$	1,306
Natural gas commodity	—		1,016		—		1,016		(10		)	1,006
Total current derivative liabilities	$	446	$	5,301	$	6	$	5,753	$	(3,441	)	2,312
PPAs (a)												5,036
Current derivative instruments												$	7,348
Noncurrent derivative liabilities
Commodity trading	$	—	$	1,362	$	—	$	1,362	$	(563	)	$	799
Total noncurrent derivative liabilities	$	—	$	1,362	$	—	$	1,362	$	(563	)	799
PPAs (a)												$	2,669
Noncurrent derivative instruments												$	3,468

^(a)  During 2006, PSCo qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

^(b)  PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

There were $0.1 million of gains recognized in earnings for Level 3 commodity trading derivatives in the three and six months ended June 30, 2018. There were immaterial gains and losses recognized in earnings for Level 3 commodity trading derivatives in the three and six months ended June 30, 2017.

PSCo recognizes transfers between levels as of the beginning of each period. There were no transfers of amounts between levels for derivative instruments for the three and six months ended June 30, 2018 and 2017.

Fair Value of Long-Term Debt

As of June 30, 2018 and Dec. 31, 2017, other financial instruments for which the carrying amount did not equal fair value were as follows:

	June 30, 2018				Dec. 31, 2017
(Thousands of Dollars)	Carrying Amount		Fair Value		Carrying Amount		Fair Value
Long-term debt, including current portion	$	5,300,062	$	5,464,543	$	4,608,275	$	5,024,840

The fair value of PSCo’s long-term debt is estimated based on recent trades and observable spreads from benchmark interest rates for similar securities. The fair value estimates are based on information available to management as of June 30, 2018 and Dec. 31, 2017, and given the observability of the inputs to these estimates, the fair values presented for long-term debt have been assigned a Level 2.

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Other Income, Net

6 Months Ended

Jun. 30, 2018

Other Income and Expenses [Abstract]

Other Income, Net

Other Income, Net

Other income, net consisted of the following:

	Three Months Ended June 30						Six Months Ended June 30
(Thousands of Dollars)	2018			2017			2018			2017
Interest income	$	508		$	609		$	372		$	984
Other nonoperating income	483			1,326			956			4,747
Insurance policy expense	(73		)	(103		)	(150		)	(182		)
Benefits non-service cost	(119		)	(513		)	(148		)	(1,026		)
Other income, net	$	799		$	1,319		$	1,030		$	4,523

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Segment Information

6 Months Ended

Jun. 30, 2018

Segment Reporting [Abstract]

Segment Information

Segment Information

Operating results from the regulated electric utility and regulated natural gas utility are each separately and regularly reviewed by PSCo’s chief operating decision maker. PSCo evaluates performance based on profit or loss generated from the product or service provided. These segments are managed separately because the revenue streams are dependent upon regulated rate recovery, which is separately determined for each segment.

PSCo has the following reportable segments: regulated electric utility, regulated natural gas utility and all other.

• PSCo’s regulated electric utility segment generates, transmits and distributes electricity primarily in portions of Colorado. In addition, this segment includes sales for resale and provides wholesale transmission service to various entities in the United States. Regulated electric utility also includes PSCo’s commodity trading operations.

• PSCo’s regulated natural gas utility segment transports, stores and distributes natural gas primarily in portions of Colorado.

• Revenues from operating segments not included above are below the necessary quantitative thresholds and are therefore included in the all other category. Those primarily include steam revenue, appliance repair services and nonutility real estate activities.

Asset and capital expenditure information is not provided for PSCo’s reportable segments because as an integrated electric and natural gas utility, PSCo operates significant assets that are not dedicated to a specific business segment, and reporting assets and capital expenditures by business segment would require arbitrary and potentially misleading allocations which may not necessarily reflect the assets that would be required for the operation of the business segments on a stand-alone basis.

To report income from operations for regulated electric and regulated natural gas utility segments, the majority of costs are directly assigned to each segment. However, some costs, such as common depreciation, common operating and maintenance (O&M) expenses and interest expense are allocated based on cost causation allocators. A general allocator is used for certain general and administrative expenses, including office supplies, rent, property insurance and general advertising.

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other			Reconciling Eliminations			Consolidated Total
Three Months Ended June 30, 2018
Operating revenues (a)(b)	$	716,195	$	186,661	$	9,010		$	—		$	911,866
Intersegment revenues	76		79		—			(155		)	—
Total revenues	$	716,271	$	186,740	$	9,010		$	(155	)	$	911,866
Net income (loss)	$	101,956	$	20,963	$	(623	)	$	—		$	122,296

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Reconciling Eliminations			Consolidated Total
Three Months Ended June 30, 2017
Operating revenues (a)(b)	$	729,920	$	192,777	$	8,219	$	—		$	930,916
Intersegment revenues	67		40		—		(107		)	—
Total revenues	$	729,987	$	192,817	$	8,219	$	(107	)	$	930,916
Net income	$	87,403	$	12,835	$	349	$	—		$	100,587

^(a)    Operating revenues include $0 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017.

^(b)    Operating revenues include $1 million of other affiliate revenue for the three months ended June 30, 2018 and 2017.

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other			Reconciling Eliminations			Consolidated Total
Six Months Ended June 30, 2018
Operating revenues from external customers	$	1,414,469	$	550,647	$	20,048		$	—		$	1,985,164
Intersegment revenues	188		143		—			(331		)	—
Total revenues	$	1,414,657	$	550,790	$	20,048		$	(331	)	$	1,985,164
Net income (loss)	$	181,507	$	74,675	$	(164	)	$	—		$	256,018

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Reconciling Eliminations			Consolidated Total
Six Months Ended June 30, 2017
Operating revenues from external customers	$	1,441,308	$	548,913	$	21,229	$	—		$	2,011,450
Intersegment revenues	159		96		—		(255		)	—
Total revenues	$	1,441,467	$	549,009	$	21,229	$	(255	)	$	2,011,450
Net income	$	163,547	$	47,318	$	1,268	$	—		$	212,133

^(a)    Operating revenues include $0 million and $1 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017.

^(b)    Operating revenues include $2 million of other affiliate revenue for the six months ended June 30, 2018 and 2017.

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Benefit Plans and Other Postretirement Benefits

6 Months Ended

Jun. 30, 2018

Retirement Benefits [Abstract]

Benefit Plans and Other Postretirement Benefits

Benefit Plans and Other Postretirement Benefits

Components of Net Periodic Benefit Cost (Credit)

	Three Months Ended June 30
	2018			2017			2018			2017
(Thousands of Dollars)	Pension Benefits						Postretirement Health Care Benefits
Service cost	$	7,271		$	6,820		$	152		$	192
Interest cost (a)	11,814			12,640			3,748			4,191
Expected return on plan assets (a)	(17,130		)	(17,134		)	(5,674		)	(5,476		)
Amortization of prior service credit (a)	(845		)	(803		)	(1,544		)	(1,562		)
Amortization of net loss (a)	7,815			7,089			1,021			961
Net periodic benefit cost (credit)	8,925			8,612			(2,297		)	(1,694		)
Credits not recognized due to the effects of regulation	895			426			—			—
Net benefit cost (credit) recognized for financial reporting	$	9,820		$	9,038		$	(2,297	)	$	(1,694	)

	Six Months Ended June 30
	2018			2017			2018			2017
(Thousands of Dollars)	Pension Benefits						Postretirement Health Care Benefits
Service cost	$	14,542		$	13,640		$	304		$	384
Interest cost (a)	23,628			25,280			7,497			8,382
Expected return on plan assets (a)	(34,260		)	(34,268		)	(11,349		)	(10,952		)
Amortization of prior service credit (a)	(1,690		)	(1,606		)	(3,089		)	(3,124		)
Amortization of net loss (a)	15,630			14,178			2,042			1,922
Net periodic benefit cost (credit)	17,850			17,224			(4,595		)	(3,388		)
Credits not recognized due to the effects of regulation	2,370			1,162			—			—
Net benefit cost (credit) recognized for financial reporting	$	20,220		$	18,386		$	(4,595	)	$	(3,388	)

^(a)   The components of net periodic cost other than the service cost component are included in the line item “other income, net” in the income statement or capitalized on the balance sheet as a regulatory asset.

In January 2018, contributions of $150 million were made across four of Xcel Energy’s pension plans, of which $22.0 million was attributable to PSCo. Xcel Energy does not expect additional pension contributions during 2018.

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Other Comprehensive Income

6 Months Ended

Jun. 30, 2018

Stockholders' Equity Note [Abstract]

Other Comprehensive Income

Other Comprehensive Income (Loss)

Changes in accumulated other comprehensive loss, net of tax, for the three and six months ended June 30, 2018 and 2017 were as follows:

	Three Months Ended June 30, 2018
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at April 1	$	(26,165	)	$	(265	)	$	(26,430	)
Losses reclassified from net accumulated other comprehensive loss	303			2			305
Net current period other comprehensive income	303			2			305
Accumulated other comprehensive loss at June 30	$	(25,862	)	$	(263	)	$	(26,125	)

	Three Months Ended June 30, 2017
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at April 1	$	(22,534	)	$	(219	)	$	(22,753	)
Losses reclassified from net accumulated other comprehensive loss	250			1			251
Net current period other comprehensive income	250			1			251
Accumulated other comprehensive loss at June 30	$	(22,284	)	$	(218	)	$	(22,502	)

	Six Months Ended June 30, 2018
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at Jan. 1	$	(26,465	)	$	(267	)	$	(26,732	)
Losses reclassified from net accumulated other comprehensive loss	603			4			607
Net current period other comprehensive income	603			4			607
Accumulated other comprehensive loss at June 30	$	(25,862	)	$	(263	)	$	(26,125	)

	Six Months Ended June 30, 2017
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at Jan. 1	$	(22,780	)	$	(220	)	$	(23,000	)
Losses reclassified from net accumulated other comprehensive loss	496			2			498
Net current period other comprehensive income	496			2			498
Accumulated other comprehensive loss at June 30	$	(22,284	)	$	(218	)	$	(22,502	)

Reclassifications from accumulated other comprehensive loss for the three and six months ended June 30, 2018 and 2017 were as follows:

	Amounts Reclassified from Accumulated Other Comprehensive Loss
(Thousands of Dollars)	Three Months Ended June 30, 2018				Three Months Ended June 30, 2017
Losses on cash flow hedges:
Interest rate derivatives	$	402		(a)	$	403		(a)
Total, pre-tax	402				403
Tax benefit	(99		)		(153		)
Total, net of tax	303				250
Defined benefit pension and postretirement losses:
Amortization of net loss	2			(b)	2			(b)
Total, pre-tax	2				2
Tax benefit	—				(1		)
Total, net of tax	2				1
Total amounts reclassified, net of tax	$	305			$	251

	Amounts Reclassified from Accumulated Other Comprehensive Loss
(Thousands of Dollars)	Six Months Ended June 30, 2018				Six Months Ended June 30, 2017
Losses on cash flow hedges:
Interest rate derivatives	$	800		(a)	$	801		(a)
Total, pre-tax	800				801
Tax benefit	(197		)		(305		)
Total, net of tax	$	603			$	496
Defined benefit pension and postretirement losses:
Amortization of net loss	$	4		(b)	$	4		(b)
Total, pre-tax	4				4
Tax benefit	—				(2		)
Total, net of tax	4				2
Total amounts reclassified, net of tax	$	607			$	498

^(a)  Included in interest charges.

^(b)  Included in the computation of net periodic pension and postretirement benefit costs. See Note 11 to the consolidated financial statements for details regarding these benefit plans.

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Revenues

6 Months Ended

Jun. 30, 2018

Revenue from Contract with Customer [Abstract]

Revenue from Contract with Customer [Text Block]

Revenues

PSCo principally generates revenue from the generation, transmission, distribution and sale of electricity and the transportation, distribution and sale of natural gas to wholesale and retail customers. Performance obligations related to the sale of energy are satisfied as energy is delivered to customers. PSCo recognizes revenue in an amount that corresponds directly to the price of the energy delivered to the customer. The measurement of energy sales to customers is generally based on the reading of their meters, which occurs on a systematic basis throughout the month. At the end of each month, amounts of energy delivered to customers since the date of the last meter reading are estimated, and the corresponding unbilled revenue is recognized. Contract terms are generally short-term in nature, and as such PSCo does not recognize a separate financing component of its collections from customers. PSCo presents its revenues net of any excise or other fiduciary-type taxes or fees.

PSCo has various rate-adjustment mechanisms in place that provide for the recovery of natural gas, electric fuel and purchased energy costs. These cost-adjustment tariffs may increase or decrease the level of revenue collected from customers and are revised periodically for differences between the total amount collected under the clauses and the costs incurred. When applicable, under governing regulatory commission rate orders, fuel cost over-recoveries (the excess of fuel revenue billed to customers over fuel costs incurred) are deferred as regulatory liabilities and under-recoveries (the excess of fuel costs incurred over fuel revenues billed to customers) are deferred as regulatory assets.

Certain rate rider mechanisms qualify as alternative revenue programs under GAAP. These mechanisms arise from costs imposed upon the utility by action of a regulator or legislative body related to an environmental, public safety or other mandate. When certain criteria are met (including collection within 24 months), revenue is recognized equal to the revenue requirement, which may include return on rate base items and incentives. The mechanisms are revised periodically for differences between the total amount collected and the revenue recognized, which may increase or decrease the level of revenue collected from customers. Alternative revenue is recorded on a gross basis and is disclosed separate from revenue from contracts with customers in the period earned.

In the following tables, revenue is classified by the type of goods/services rendered and market/customer type. The tables also reconcile revenue to the reportable segments.

	Three Months Ended June 30, 2018
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	222,687	$	104,000	$	2,560	$	329,247
Commercial and industrial (C&I)	383,707		38,904		5,326		427,937
Other	11,455		—		—		11,455
Total retail	617,849		142,904		7,886		768,639
Wholesale	36,148		—		—		36,148
Transmission	13,159		—		—		13,159
Other	14,696		18,845		—		33,541
Total revenue from contracts with customers	681,852		161,749		7,886		851,487
Alternative revenue and other	34,343		24,912		1,124		60,379
Total revenues	$	716,195	$	186,661	$	9,010	$	911,866

	Three Months Ended June 30, 2017
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	225,921	$	117,393	$	2,639	$	345,953
C&I	392,280		45,032		4,457		441,769
Other	11,711		—		—		11,711
Total retail	629,912		162,425		7,096		799,433
Wholesale	41,579		—		—		41,579
Transmission	13,180		—		—		13,180
Other	16,837		20,040		—		36,877
Total revenue from contracts with customers	701,508		182,465		7,096		891,069
Alternative revenue and other	28,412		10,312		1,123		39,847
Total revenues	$	729,920	$	192,777	$	8,219	$	930,916

	Six Months Ended June 30, 2018
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	450,336	$	331,746	$	5,256	$	787,338
C&I	726,933		124,932		12,485		864,350
Other	23,631		—		60		23,691
Total retail	1,200,900		456,678		17,801		1,675,379
Wholesale	84,038		—		—		84,038
Transmission	25,411		—		—		25,411
Other	33,527		43,774		—		77,301
Total revenue from contracts with customers	1,343,876		500,452		17,801		1,862,129
Alternative revenue and other	70,593		50,195		2,247		123,035
Total revenues	$	1,414,469	$	550,647	$	20,048	$	1,985,164

	Six Months Ended June 30, 2017
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	460,355	$	343,303	$	5,203	$	808,861
C&I	751,977		131,274		13,717		896,968
Other	24,386		—		63		24,449
Total retail	1,236,718		474,577		18,983		1,730,278
Wholesale	85,155		—		—		85,155
Transmission	27,819		—		—		27,819
Other	33,532		42,650		—		76,182
Total revenue from contracts with customers	1,383,224		517,227		18,983		1,919,434
Alternative revenue and other	58,084		31,686		2,246		92,016
Total revenues	$	1,441,308	$	548,913	$	21,229	$	2,011,450

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Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2018

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Accounts receivable, net
Accounts receivable	$	290,832		$	314,009
Less allowance for bad debts	(18,539		)	(19,606		)
	$	272,293		$	294,403

Inventories

(Thousands of Dollars)	June 30, 2018		Dec. 31, 2017
Inventories
Materials and supplies	$	70,940	$	68,940
Fuel	62,532		73,893
Natural gas	36,248		71,656
	$	169,720	$	214,489

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Property, plant and equipment, net
Electric plant	$	12,780,963		$	12,627,592
Natural gas plant	4,211,977			4,102,075
Common and other property	1,040,638			1,022,333
Plant to be retired (a)	10,306			10,949
Construction work in progress	1,444,140			1,014,338
Total property, plant and equipment	19,488,024			18,777,287
Less accumulated depreciation	(4,946,946		)	(4,751,536		)
	$	14,541,078		$	14,025,751

^(a)  In the third quarter of 2017, PSCo early retired Valmont Unit 5 and converted Cherokee Unit 4 from a coal-fueled generating facility to natural gas. PSCo also expects Craig Unit 1 to be early retired in approximately 2025. Amounts are presented net of accumulated depreciation.

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Income Taxes (Tables)

6 Months Ended

Jun. 30, 2018

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]

Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences:

	Six Months Ended June 30
	2018		2017
Federal statutory rate	21.0	%	35.0	%
State tax, net of federal tax effect	3.7		3.0
Increases (decreases) in tax from:
Regulatory differences - ARAM (a)	(3.2	)	(0.1	)
Regulatory differences - ARAM deferral (b)	3.0		—
Regulatory differences - other utility plant items	(1.3	)	(0.9	)
Tax credits, net of federal income tax expense	(1.0	)	(0.7	)
Other, net	0.3		0.1
Effective income tax rate	22.5	%	36.4	%

^(a)   The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.

^(b) As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the Tax Cuts and Jobs Act

(TCJA)), the ARAM deferral may decrease during the year, which would result in a reduction to tax expense with a corresponding reduction to revenue.

Summary of Statute of Limitations Applicable to Open Tax Years [Table Text Block]

PSCO is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows:

Tax Year(s)	Expiration
2009 - 2011	December 2018
2012 - 2014	October 2019
2015	September 2019
2016	September 2020

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the amount of unrecognized tax benefit is as follows:

(Millions of Dollars)	June 30, 2018		Dec. 31, 2017
Unrecognized tax benefit — Permanent tax positions	$	4.4	$	4.0
Unrecognized tax benefit — Temporary tax positions	5.0		6.1
Total unrecognized tax benefit	$	9.4	$	10.1

Tax Benefits Associated with NOL and Tax Credit Carryforwards

The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows:

(Millions of Dollars)	June 30, 2018			Dec. 31, 2017
NOL and tax credit carryforwards	$	(4.7	)	$	(4.0	)

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Rate Matters (Tables)

6 Months Ended

Jun. 30, 2018

Public Utilities, General Disclosures [Abstract]

Colorado 2017 Multi-Year Gas Rate Case - CPUC Decision [Table Text Block]

The estimated impact of the CPUC’s decision is presented below:

(Millions of Dollars)	Estimated Impact of the CPUC’s Decision
Filed 2018 revenue request based on a FTY	$	63
Impact of the change in test year	5
PSCo’s deficiency based on a 2016 HTY - year-end rate base	68
Adjustments:
ROE at 9.35 percent	(9		)
Equity ratio of 54.6 percent	(2		)
Change in amortization period for certain regulatory assets, including a debt return	(6		)
Loss of return on prepaid pension and retiree medical	(4		)
Change from 2016 year-end to average rate base	(5		)
Other, net	5
Total adjustments	(21		)
Total rate increase, prior to the TCJA impacts	$	47

Colorado 2017 Multi-Year Electric Rate Case [Table Text Block]

Revenue Request (Millions of Dollars)	2018		2019		2020		2021		Total
Revenue request	$	74	$	75	$	60	$	36	$	245
Clean Air Clean Jobs Act (CACJA) rider conversion to base rates	90		—		—		—		90
Transmission Cost Adjustment (TCA) rider conversion to base rates	43		—		—		—		43
Total	$	207	$	75	$	60	$	36	$	378
Expected year-end rate base (billions of dollars)	$	6.8	$	7.1	$	7.3	$	7.4

Colorado 2017 Multi-Year Gas Rate Case [Table Text Block]

The request, detailed below, was based on FTYs, a 10.0 percent ROE and an equity ratio of 55.25 percent.

Revenue Request (Millions of Dollars)	2018		2019		2020		Total
Revenue request	$	63	$	33	$	43	$	139
Pipeline System Integrity Adjustment (PSIA) rider conversion to base rates (a)	—		94		—		94
Total	$	63	$	127	$	43	$	233
Expected year-end rate base (billions of dollars) (b)	$	1.5	$	2.3	$	2.4

^(a)   The roll-in of PSIA rider revenue into base rates will not have an impact on customer bills or revenue as these costs are already being recovered through the rider. The recovery of incremental PSIA related investments in 2019 and 2020 are included in the base rate request.

^(b)   The additional rate base in 2019 predominantly reflects the roll-in of capital associated with the PSIA rider.

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Borrowings and Other Financing Instruments (Tables)

6 Months Ended

Jun. 30, 2018

Borrowings and Other Financing Instruments [Abstract]

Short-Term Borrowings

Commercial paper outstanding for PSCo was as follows:

(Amounts in Millions, Except Interest Rates)	Three Months Ended June 30, 2018			Year Ended Dec. 31, 2017
Borrowing limit	$	700		$	700
Amount outstanding at period end	—			—
Average amount outstanding	84			54
Maximum amount outstanding	257			268
Weighted average interest rate, computed on a daily basis	2.17		%	1.08		%
Weighted average interest rate at period end	N/A			N/A

Credit Facilities

At June 30, 2018, PSCo had the following committed credit facility available (in millions of dollars):

Credit Facility (a)		Drawn(b)		Available
$	700	$	4	$	696

^(a)    This credit facility expires in June 2021.

^(b)    Includes outstanding commercial paper and letters of credit.

Money Pool

Borrowings and Other Financing Instruments [Abstract]

Short-Term Borrowings

Money pool borrowings for PSCo were as follows:

(Amounts in Millions, Except Interest Rates)	Three Months Ended June 30, 2018			Year Ended Dec. 31, 2017
Borrowing limit	$	250		$	250
Amount outstanding at period end	—			—
Average amount outstanding	60			—
Maximum amount outstanding	156			20
Weighted average interest rate, computed on a daily basis	1.84		%	0.92		%
Weighted average interest rate at period end	N/A			N/A

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Fair Value of Financial Assets and Liabilities (Tables)

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Gross Notional Amounts of Commodity Forwards and Options

The following table details the gross notional amounts of commodity forwards and options at June 30, 2018 and Dec. 31, 2017:

(Amounts in Thousands)(a)(b)	June 30, 2018	Dec. 31, 2017
Megawatt hours of electricity	20,813	22,260
Million British thermal units of natural gas	18,798	13,410

^(a)  Amounts are not reflective of net positions in the underlying commodities.

^(b)  Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income

The following tables detail the impact of derivative activity during the three months ended June 30, 2018 and 2017 on accumulated other comprehensive loss, regulatory assets and liabilities, and income:

	Three Months Ended June 30, 2018
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)		Pre-Tax Gains Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	402	(a)	$	—	$	—
Total	$	—	$	—		$	402		$	—	$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—	$	200	(b)
Natural gas commodity	—		(249		)	—			—		—		(c)
Total	$	—	$	(249	)	$	—		$	—	$	200

	Six Months Ended June 30, 2018
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:						Pre-Tax (Gains) Losses Recognized During the Period in Income
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	800	(a)	$	—		$	—
Total	$	—	$	—		$	800		$	—		$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—		$	724		(b)
Natural gas commodity	—		(420		)	—			2,749		(c)	(1,581		)	(c)
Total	$	—	$	(420	)	$	—		$	2,749		$	(857	)

	Three Months Ended June 30, 2017
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)		Pre-Tax Losses Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	403	(a)	$	—	$	—
Total	$	—	$	—		$	403		$	—	$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—	$	(192	)	(b)
Natural gas commodity	—		(1,621		)	—			—		—			(c)
Total	$	—	$	(1,621	)	$	—		$	—	$	(192	)

	Six Months Ended June 30, 2017
	Pre-Tax Fair Value Losses Recognized During the Period in:					Pre-Tax Losses Reclassified into Income During the Period from:
(Thousands of Dollars)	Accumulated Other Comprehensive Loss		Regulatory (Assets) and Liabilities			Accumulated Other Comprehensive Loss			Regulatory Assets and (Liabilities)			Pre-Tax Gains (Losses) Recognized During the Period in Income
Derivatives designated as cash flow hedges
Interest rate	$	—	$	—		$	801	(a)	$	—		$	—
Total	$	—	$	—		$	801		$	—		$	—
Other derivative instruments
Commodity trading	$	—	$	—		$	—		$	—		$	187		(b)
Natural gas commodity	—		(7,008		)	—			282		(c)	(2,990		)	(c)
Total	$	—	$	(7,008	)	$	—		$	282		$	(2,803	)

^(a)  Amounts are recorded to interest charges.

^(b)  Amounts are recorded to interest charges. Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue as appropriate.

^(c)  Certain derivatives are utilized to mitigate natural gas price risk for electric generation and are recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. Amounts for the three and six months endedJune 30, 2018 included no settlement gains or losses and $1.2 million of settlement losses, respectively. Amounts for the three and six months ended June 30, 2017 included no settlement gains or losses and $0.9 million of settlement gains, respectively. The remaining derivative settlement gains and losses for the six months endedJune 30, 2018 and 2017 relate to natural gas operations and are recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

Derivative Assets and Liabilities Measured at Fair Value on a Recurring Basis by Hierarchy Level

Recurring Fair Value Measurements—The following table presents, for each of the fair value hierarchy levels, PSCo’s assets and liabilities measured at fair value on a recurring basis at June 30, 2018:

	June 30, 2018
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative assets
Other derivative instruments:
Commodity trading	$	332	$	12,812	$	118	$	13,262	$	(9,729	)	$	3,533
Natural gas commodity	—		919		—		919		—			919
Total current derivative assets	$	332	$	13,731	$	118	$	14,181	$	(9,729	)	4,452
PPAs (a)												888
Current derivative instruments												$	5,340
Noncurrent derivative assets
Other derivative instruments:
Commodity trading	$	1	$	2,715	$	—	$	2,716	$	—		$	2,716
Total noncurrent derivative assets	$	1	$	2,715	$	—	$	2,716	$	—		2,716
PPAs (a)												—
Noncurrent derivative instruments												$	2,716

	June 30, 2018
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative liabilities
Other derivative instruments:
Commodity trading	$	333	$	12,409	$	—	$	12,742	$	(12,474	)	$	268
Total current derivative liabilities	$	333	$	12,409	$	—	$	12,742	$	(12,474	)	268
PPAs (a)												5,160
Current derivative instruments												$	5,428
Noncurrent derivative liabilities
Other derivative instruments:
Commodity trading	$	—	$	2,382	$	—	$	2,382	$	—		$	2,382
Total noncurrent derivative liabilities	$	—	$	2,382	$	—	$	2,382	$	—		2,382
PPAs (a)												89
Noncurrent derivative instruments												$	2,471

^(a)  During 2006, PSCo qualified these contracts under the normal purchase exception. Based onthis qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

^(b)  PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at June 30, 2018. At June 30, 2018, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $2.7 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

The following table presents, for each of the fair value hierarchy levels, PSCo’s assets and liabilities measured at fair value on a recurring basis at Dec. 31, 2017:

	Dec. 31, 2017
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative assets
Other derivative instruments:
Commodity trading	$	528	$	4,488	$	12	$	5,028	$	(3,554	)	$	1,474
Natural gas commodity	—		18		—		18		(10		)	8
Total current derivative assets	$	528	$	4,506	$	12	$	5,046	$	(3,564	)	1,482
PPAs (a)												1,715
Current derivative instruments												$	3,197
Noncurrent derivative assets
Other derivative instruments:
Commodity trading	$	—	$	1,541	$	—	$	1,541	$	(563	)	$	978
Total noncurrent derivative assets	$	—	$	1,541	$	—	$	1,541	$	(563	)	978
PPAs (a)												31
Noncurrent derivative instruments												$	1,009

	Dec. 31, 2017
	Fair Value						Fair Value Total		Counterparty Netting(b)
(Thousands of Dollars)	Level 1		Level 2		Level 3							Total
Current derivative liabilities
Other derivative instruments:
Commodity trading	$	446	$	4,285	$	6	$	4,737	$	(3,431	)	$	1,306
Natural gas commodity	—		1,016		—		1,016		(10		)	1,006
Total current derivative liabilities	$	446	$	5,301	$	6	$	5,753	$	(3,441	)	2,312
PPAs (a)												5,036
Current derivative instruments												$	7,348
Noncurrent derivative liabilities
Commodity trading	$	—	$	1,362	$	—	$	1,362	$	(563	)	$	799
Total noncurrent derivative liabilities	$	—	$	1,362	$	—	$	1,362	$	(563	)	799
PPAs (a)												$	2,669
Noncurrent derivative instruments												$	3,468

^(a)  During 2006, PSCo qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

^(b)  PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

Carrying Amount and Fair Value of Long-term Debt

As of June 30, 2018 and Dec. 31, 2017, other financial instruments for which the carrying amount did not equal fair value were as follows:

	June 30, 2018				Dec. 31, 2017
(Thousands of Dollars)	Carrying Amount		Fair Value		Carrying Amount		Fair Value
Long-term debt, including current portion	$	5,300,062	$	5,464,543	$	4,608,275	$	5,024,840

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Other Income, Net (Tables)

6 Months Ended

Jun. 30, 2018

Other Income and Expenses [Abstract]

Other Income, Net

Other income, net consisted of the following:

	Three Months Ended June 30						Six Months Ended June 30
(Thousands of Dollars)	2018			2017			2018			2017
Interest income	$	508		$	609		$	372		$	984
Other nonoperating income	483			1,326			956			4,747
Insurance policy expense	(73		)	(103		)	(150		)	(182		)
Benefits non-service cost	(119		)	(513		)	(148		)	(1,026		)
Other income, net	$	799		$	1,319		$	1,030		$	4,523

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Segment Information (Tables)

6 Months Ended

Jun. 30, 2018

Segment Reporting [Abstract]

Results from Operations by Reportable Segment

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other			Reconciling Eliminations			Consolidated Total
Three Months Ended June 30, 2018
Operating revenues (a)(b)	$	716,195	$	186,661	$	9,010		$	—		$	911,866
Intersegment revenues	76		79		—			(155		)	—
Total revenues	$	716,271	$	186,740	$	9,010		$	(155	)	$	911,866
Net income (loss)	$	101,956	$	20,963	$	(623	)	$	—		$	122,296

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Reconciling Eliminations			Consolidated Total
Three Months Ended June 30, 2017
Operating revenues (a)(b)	$	729,920	$	192,777	$	8,219	$	—		$	930,916
Intersegment revenues	67		40		—		(107		)	—
Total revenues	$	729,987	$	192,817	$	8,219	$	(107	)	$	930,916
Net income	$	87,403	$	12,835	$	349	$	—		$	100,587

^(a)    Operating revenues include $0 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017.

^(b)    Operating revenues include $1 million of other affiliate revenue for the three months ended June 30, 2018 and 2017.

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other			Reconciling Eliminations			Consolidated Total
Six Months Ended June 30, 2018
Operating revenues from external customers	$	1,414,469	$	550,647	$	20,048		$	—		$	1,985,164
Intersegment revenues	188		143		—			(331		)	—
Total revenues	$	1,414,657	$	550,790	$	20,048		$	(331	)	$	1,985,164
Net income (loss)	$	181,507	$	74,675	$	(164	)	$	—		$	256,018

(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Reconciling Eliminations			Consolidated Total
Six Months Ended June 30, 2017
Operating revenues from external customers	$	1,441,308	$	548,913	$	21,229	$	—		$	2,011,450
Intersegment revenues	159		96		—		(255		)	—
Total revenues	$	1,441,467	$	549,009	$	21,229	$	(255	)	$	2,011,450
Net income	$	163,547	$	47,318	$	1,268	$	—		$	212,133

^(a)    Operating revenues include $0 million and $1 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017.

^(b)    Operating revenues include $2 million of other affiliate revenue for the six months ended June 30, 2018 and 2017.

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Benefit Plans and Other Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2018

Retirement Benefits [Abstract]

Components of Net Periodic Benefit Cost (Credit)

Components of Net Periodic Benefit Cost (Credit)

	Three Months Ended June 30
	2018			2017			2018			2017
(Thousands of Dollars)	Pension Benefits						Postretirement Health Care Benefits
Service cost	$	7,271		$	6,820		$	152		$	192
Interest cost (a)	11,814			12,640			3,748			4,191
Expected return on plan assets (a)	(17,130		)	(17,134		)	(5,674		)	(5,476		)
Amortization of prior service credit (a)	(845		)	(803		)	(1,544		)	(1,562		)
Amortization of net loss (a)	7,815			7,089			1,021			961
Net periodic benefit cost (credit)	8,925			8,612			(2,297		)	(1,694		)
Credits not recognized due to the effects of regulation	895			426			—			—
Net benefit cost (credit) recognized for financial reporting	$	9,820		$	9,038		$	(2,297	)	$	(1,694	)

	Six Months Ended June 30
	2018			2017			2018			2017
(Thousands of Dollars)	Pension Benefits						Postretirement Health Care Benefits
Service cost	$	14,542		$	13,640		$	304		$	384
Interest cost (a)	23,628			25,280			7,497			8,382
Expected return on plan assets (a)	(34,260		)	(34,268		)	(11,349		)	(10,952		)
Amortization of prior service credit (a)	(1,690		)	(1,606		)	(3,089		)	(3,124		)
Amortization of net loss (a)	15,630			14,178			2,042			1,922
Net periodic benefit cost (credit)	17,850			17,224			(4,595		)	(3,388		)
Credits not recognized due to the effects of regulation	2,370			1,162			—			—
Net benefit cost (credit) recognized for financial reporting	$	20,220		$	18,386		$	(4,595	)	$	(3,388	)

^(a)   The components of net periodic cost other than the service cost component are included in the line item “other income, net” in the income statement or capitalized on the balance sheet as a regulatory asset.

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Other Comprehensive Income (Tables)

6 Months Ended

Jun. 30, 2018

Stockholders' Equity Note [Abstract]

Changes in Accumulated Other Comprehensive Loss, Net of Tax

Changes in accumulated other comprehensive loss, net of tax, for the three and six months ended June 30, 2018 and 2017 were as follows:

	Three Months Ended June 30, 2018
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at April 1	$	(26,165	)	$	(265	)	$	(26,430	)
Losses reclassified from net accumulated other comprehensive loss	303			2			305
Net current period other comprehensive income	303			2			305
Accumulated other comprehensive loss at June 30	$	(25,862	)	$	(263	)	$	(26,125	)

	Three Months Ended June 30, 2017
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at April 1	$	(22,534	)	$	(219	)	$	(22,753	)
Losses reclassified from net accumulated other comprehensive loss	250			1			251
Net current period other comprehensive income	250			1			251
Accumulated other comprehensive loss at June 30	$	(22,284	)	$	(218	)	$	(22,502	)

	Six Months Ended June 30, 2018
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at Jan. 1	$	(26,465	)	$	(267	)	$	(26,732	)
Losses reclassified from net accumulated other comprehensive loss	603			4			607
Net current period other comprehensive income	603			4			607
Accumulated other comprehensive loss at June 30	$	(25,862	)	$	(263	)	$	(26,125	)

	Six Months Ended June 30, 2017
(Thousands of Dollars)	Gains and Losses on Cash Flow Hedges			Defined Benefit and Postretirement Items			Total
Accumulated other comprehensive loss at Jan. 1	$	(22,780	)	$	(220	)	$	(23,000	)
Losses reclassified from net accumulated other comprehensive loss	496			2			498
Net current period other comprehensive income	496			2			498
Accumulated other comprehensive loss at June 30	$	(22,284	)	$	(218	)	$	(22,502	)

Reclassifications out of Accumulated Other Comprehensive Loss

Reclassifications from accumulated other comprehensive loss for the three and six months ended June 30, 2018 and 2017 were as follows:

	Amounts Reclassified from Accumulated Other Comprehensive Loss
(Thousands of Dollars)	Three Months Ended June 30, 2018				Three Months Ended June 30, 2017
Losses on cash flow hedges:
Interest rate derivatives	$	402		(a)	$	403		(a)
Total, pre-tax	402				403
Tax benefit	(99		)		(153		)
Total, net of tax	303				250
Defined benefit pension and postretirement losses:
Amortization of net loss	2			(b)	2			(b)
Total, pre-tax	2				2
Tax benefit	—				(1		)
Total, net of tax	2				1
Total amounts reclassified, net of tax	$	305			$	251

	Amounts Reclassified from Accumulated Other Comprehensive Loss
(Thousands of Dollars)	Six Months Ended June 30, 2018				Six Months Ended June 30, 2017
Losses on cash flow hedges:
Interest rate derivatives	$	800		(a)	$	801		(a)
Total, pre-tax	800				801
Tax benefit	(197		)		(305		)
Total, net of tax	$	603			$	496
Defined benefit pension and postretirement losses:
Amortization of net loss	$	4		(b)	$	4		(b)
Total, pre-tax	4				4
Tax benefit	—				(2		)
Total, net of tax	4				2
Total amounts reclassified, net of tax	$	607			$	498

^(a)  Included in interest charges.

^(b) 

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Revenues (Tables)

6 Months Ended

Jun. 30, 2018

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue [Table Text Block]

In the following tables, revenue is classified by the type of goods/services rendered and market/customer type. The tables also reconcile revenue to the reportable segments.

	Three Months Ended June 30, 2018
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	222,687	$	104,000	$	2,560	$	329,247
Commercial and industrial (C&I)	383,707		38,904		5,326		427,937
Other	11,455		—		—		11,455
Total retail	617,849		142,904		7,886		768,639
Wholesale	36,148		—		—		36,148
Transmission	13,159		—		—		13,159
Other	14,696		18,845		—		33,541
Total revenue from contracts with customers	681,852		161,749		7,886		851,487
Alternative revenue and other	34,343		24,912		1,124		60,379
Total revenues	$	716,195	$	186,661	$	9,010	$	911,866

	Three Months Ended June 30, 2017
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	225,921	$	117,393	$	2,639	$	345,953
C&I	392,280		45,032		4,457		441,769
Other	11,711		—		—		11,711
Total retail	629,912		162,425		7,096		799,433
Wholesale	41,579		—		—		41,579
Transmission	13,180		—		—		13,180
Other	16,837		20,040		—		36,877
Total revenue from contracts with customers	701,508		182,465		7,096		891,069
Alternative revenue and other	28,412		10,312		1,123		39,847
Total revenues	$	729,920	$	192,777	$	8,219	$	930,916

	Six Months Ended June 30, 2018
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	450,336	$	331,746	$	5,256	$	787,338
C&I	726,933		124,932		12,485		864,350
Other	23,631		—		60		23,691
Total retail	1,200,900		456,678		17,801		1,675,379
Wholesale	84,038		—		—		84,038
Transmission	25,411		—		—		25,411
Other	33,527		43,774		—		77,301
Total revenue from contracts with customers	1,343,876		500,452		17,801		1,862,129
Alternative revenue and other	70,593		50,195		2,247		123,035
Total revenues	$	1,414,469	$	550,647	$	20,048	$	1,985,164

	Six Months Ended June 30, 2017
(Thousands of Dollars)	Electric		Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	460,355	$	343,303	$	5,203	$	808,861
C&I	751,977		131,274		13,717		896,968
Other	24,386		—		63		24,449
Total retail	1,236,718		474,577		18,983		1,730,278
Wholesale	85,155		—		—		85,155
Transmission	27,819		—		—		27,819
Other	33,532		42,650		—		76,182
Total revenue from contracts with customers	1,383,224		517,227		18,983		1,919,434
Alternative revenue and other	58,084		31,686		2,246		92,016
Total revenues	$	1,441,308	$	548,913	$	21,229	$	2,011,450

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Accounting Pronouncements (Details) - Accounting Standards Update 2017-07 $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)
Operating and Maintenance Expense
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net benefit cost (credit) recognized for financial reporting	$ (1.0)
Other Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net benefit cost (credit) recognized for financial reporting	$ 1.0

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Selected Balance Sheet Data (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts receivable, net
Accounts receivable	$ 290,832	$ 314,009
Less allowance for bad debts	(18,539)	(19,606)
Accounts receivable, net	$ 272,293	$ 294,403

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Selected Balance Sheet Data Balance Sheet Related Disclosures, Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Inventories	$ 169,720	$ 214,489
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	70,940	68,940
Fuel
Public Utilities, Inventory [Line Items]
Inventories	62,532	73,893
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 36,248	$ 71,656

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Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 19,488,024	$ 18,777,287
Less accumulated depreciation		(4,946,946)	(4,751,536)
Property, plant and equipment, net		14,541,078	14,025,751
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		12,780,963	12,627,592
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		4,211,977	4,102,075
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,040,638	1,022,333
Plant to be retired
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	[1]	10,306	10,949
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 1,444,140	$ 1,014,338

[1] (a) In the third quarter of 2017, PSCo early retired Valmont Unit 5 and converted Cherokee Unit 4 from a coal-fueled generating facility to natural gas. PSCo also expects Craig Unit 1 to be early retired in approximately 2025. Amounts are presented net of accumulated depreciation.

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Income Taxes (Details) - USD ($)	6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2015	Dec. 31, 2012	Dec. 31, 2017
Income Tax Examination [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	3.70%	3.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	(3.20%)	(0.10%)
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent	3.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	(1.30%)	(0.90%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	(1.00%)	(0.70%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.30%	0.10%
Effective Income Tax Rate Reconciliation, Percent	22.50%	36.40%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions	$ 4,400,000				$ 4,000,000
Unrecognized tax benefit — Temporary tax positions	5,000,000				6,100,000
Total unrecognized tax benefit	9,400,000				10,100,000
NOL and tax credit carryforwards	(4,700,000)				(4,000,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible	8,000,000
Amounts accrued for penalties related to unrecognized tax benefits	$ 0				$ 0
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination			2012 and 2013	2010 and 2011
Year of carryback claim under examination				2009
State Jurisdiction (Colorado)
Tax Audits [Abstract]
Earliest year subject to examination	2009

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Rate Matters (Details) - USD ($) $ in Millions					1 Months Ended
		Jul. 12, 2018	May 01, 2018	Mar. 01, 2018	Jun. 30, 2018	May 31, 2018	Apr. 30, 2018	Feb. 28, 2018	Oct. 31, 2017	Jun. 30, 2017
CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Number of Years Which Rates are Requested to Increase										3 years
Public Utilities, Requested Return on Equity, Percentage										10.00%
Public Utilities, Requested Equity Capital Structure, Percentage										55.25%
Office of Consumer Council (OCC) | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended Length of Average Rate Base									13 months
PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Reduction of Provisional Rates								$ 20
Public Utilities, Requested Rate Increase (Decrease), Amount										$ 139
Public Utilities, Rate Impact										233
Annualized Provisional Rates				$ 43
PSCo | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approximate Rate Increase (Decrease), Amount									$ 245
Public Utilities, Number of Years Which Rates are Requested to Increase									4 years
Public Utilities, Requested Return on Equity, Percentage									10.00%
Public Utilities, Requested Equity Capital Structure, Percentage									55.25%
Public Utilities, Requested Rate Increase (Decrease), Amount									$ 245
Public Utilities, Rate Impact									378
PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount									74
Public Utilities, Rate Impact									207
Public Utilities, Expected Year-End Rate Base									6,800
PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount									75
Public Utilities, Rate Impact									75
Public Utilities, Expected Year-End Rate Base									7,100
PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount									60
Public Utilities, Rate Impact									60
Public Utilities, Expected Year-End Rate Base									7,300
PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount									36
Public Utilities, Rate Impact									36
Public Utilities, Expected Year-End Rate Base									7,400
PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount										63
Public Utilities, Rate Impact										63
Public Utilities, Expected Year-End Rate Base	[1]									1,500
PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount										33
Public Utilities, Rate Impact										127
Public Utilities, Expected Year-End Rate Base	[1]									2,300
PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount										43
Public Utilities, Rate Impact										43
Public Utilities, Expected Year-End Rate Base	[1]									2,400
PSCo | Colorado Public Utilities Commission [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Amount Rider Income Reduced for Tax Cuts and Jobs Act			$ 8
PSCo | Colorado Public Utilities Commission [Member] | CPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]
Public Utilities, General Disclosures [Line Items]
Tax Cuts and Jobs Act of 2017, 2018 Electric Refund of TCJA Benefits Under Settlement							$ 42
Tax Cuts and Jobs Act of 2017, Amount of Accelerated Prepaid Pension Asset Amortization Under Settlement							$ 59
Tax Cuts and Jobs Act of 2017, 2018 Electric Refund of TCJA Benefits Approved By Commission					$ 42
Tax Cuts and Jobs Act of 2017, Amount of Accelerated Prepaid Pension Asset Amortization Under Settlement Set For Hearing					$ 59
PSCo | Administrative Law Judge [Member] | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Reduction of Provisional Rates								20
Public Utilities, Recommended Length of Average Rate Base						13 months
Public Utilities, Recommended Equity Capital Structure, Percentage						54.20%
Public Utilities, Total Recommended Rate Increase (Decrease)						$ 46
Public Utilities, Preliminary Estimate for Reduction of Provisional Rates								$ 29
Public Utilities, Recommended Rate Base						$ 1,600
Public Utilities, Recommended ROE, Percentage						9.35%
Subsequent Event | PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Impact of Change in Test Year		$ 5
Public Utilities, Revised requested rate increase		$ 68
Subsequent Event | PSCo | Colorado Public Utilities Commission [Member] | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Length of Average Rate Base		13 months
Public Utilities, Approved Return on Equity, Percentage		9.35%
Public Utilities, Approved Equity Capital Structure, Percentage		54.60%
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 47
Subsequent Event | PSCo | Colorado Public Utilities Commission [Member] | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rate Increase (Decrease) from Approved ROE		(9)
Public Utilities, Rate Increase (Decrease) from Approved Equity Capital Structure		(2)
Public Utilities, Increase (Decrease) From Change in Amortization Period		(6)
Public Utilities, Increase (Decrease) From Approved Prepaid Pension and Retiree Medical Assets		(4)
Public Utilities, Increase (Decrease) From Change to Average Rate Base		(5)
Public Utilities, Increase (Decrease) Other, net		5
Public Utilities, Total Adjustments to Requested Rate Increase		(21)
Public Utilities, Total Approved Rate Increase, Before Impact of the Tax Cuts and Jobs Act		$ 47
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									90
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									90
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									0
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									0
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									0
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									43
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									43
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									0
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									0
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates									$ 0
Pipeline System Integrity Adjustment (PSIA) Rider | PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[2]									94
Pipeline System Integrity Adjustment (PSIA) Rider | PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[2]									0
Pipeline System Integrity Adjustment (PSIA) Rider | PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[2]									94
Pipeline System Integrity Adjustment (PSIA) Rider | PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[2]									$ 0

[1] The additional rate base in 2019 predominantly reflects the roll-in of capital associated with the PSIA rider.

[2] The roll-in of PSIA rider revenue into base rates will not have an impact on customer bills or revenue as these costs are already being recovered through the rider. The recovery of incremental PSIA related investments in 2019 and 2020 are included in the base rate request.

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Commitments and Contingencies, Purchased Power Agreements (Details) - Independent Power Producing Entities - MW	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Purchased Power Agreements [Abstract]
Generating capacity under long term purchased power agreements (in MW)	1,571	1,571
Purchase Power Agreement Expiration (year)	2032

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Commitments and Contingencies, Environmental Contingencies (Details) - Other MGP, Landfill, or Disposal Sites [Domain] $ in Millions	Jun. 30, 2018 USD ($) Site
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of identified MGP sites under current investigation and/or remediation | Site	4
Accrual for Environmental Loss Contingencies, Gross | $	$ 2

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Commitments and Contingencies Unrecorded Unconditional Purchase Obligation (Details) - National Ambient Air Quality Standards for Ozone [Member]	Dec. 31, 2015 Period
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of hours measured for standard	8
Former Level of Air Quality Concentrations (in parts per billion)	75
Revised Level of Air Quality Concentrations (in parts per billion)	70

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Commitments and Contingencies, Legal Contingencies (Details)	1 Months Ended
	Dec. 31, 2015
PSCo | Minimum [Member] | Line Extension Disputes
Legal Contingencies [Abstract]
Loss Contingency, Number of Plaintiffs	50

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Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Money Pool
Short-term Debt [Line Items]
Borrowing limit	$ 250,000,000	$ 250,000,000
Amount outstanding at period end	0	0
Average amount outstanding	60,000,000	0
Maximum amount outstanding	$ 156,000,000	$ 20,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.84%	0.92%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000,000	$ 700,000,000
Amount outstanding at period end	0	0
Average amount outstanding	84,000,000	54,000,000
Maximum amount outstanding	$ 257,000,000	$ 268,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.17%	1.08%

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Borrowings and Other Financing Instruments, Letters of Credit (Details) - Letter of Credit - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 4	$ 3
Letter of Credit
Line of Credit Facility [Line Items]
Term of letters of credit (in years)	1 year

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Borrowings and Other Financing Instruments, Credit Facility (Details) - Credit Facility - USD ($)		6 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Credit Facility	[1]	$ 700,000,000
Drawn	[2]	4,000,000
Available		$ 696,000,000
Maturity Date		Jun. 30, 2021
Direct advances on the credit facility outstanding		$ 0	$ 0

[1] This credit facility expires in June 2021.

[2] Includes outstanding commercial paper and letters of credit.

XML

Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) - Bonds [Member]	6 Months Ended
	Jun. 30, 2018 USD ($)
Series Due June 15, 2028 [Member]
Debt Instrument [Line Items]
Maturity Date	Jun. 15, 2028
Debt Instrument, Face Amount	$ 350,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.70%
Series Due June 15, 2048 [Member]
Debt Instrument [Line Items]
Maturity Date	Jun. 15, 2048
Debt Instrument, Face Amount	$ 350,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.10%

XML

Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) MWh in Thousands, MMBTU in Thousands, $ in Millions		Jun. 30, 2018 USD ($) MMBTU MWh Counterparty	Dec. 31, 2017 MMBTU MWh
Credit Concentration Risk
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		10
Credit Concentration Risk | Municipal or Cooperative Entities or Other Utilities [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure		9
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure		4
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 27.6
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		47.00%
Credit Concentration Risk | No Investment Grade Ratings from External Credit Rating Agencies [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure		5
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 12.2
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		21.00%
Credit Concentration Risk | Credit Quality Less Than Investment Grade [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure		1
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 5.2
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		9.00%
Interest Rate Swap
Interest Rate Derivatives [Abstract]
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months		$ (1.2)
Electric Commodity (in megawatt hours)
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Derivative, Nonmonetary Notional amount | MWh	[1],[2]	20,813	22,260
Natural Gas Commodity (in million British thermal units)
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Derivative, Nonmonetary Notional amount | MMBTU	[1],[2]	18,798	13,410

[1] Amounts are not reflective of net positions in the underlying commodities.

[2] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

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Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges		$ 0	$ 0	$ 0		$ 0
Recognized gains (losses) from fair value hedges or related hedged transactions		0	0	0		0
Designated as Hedging Instrument | Cash Flow Hedges
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0	0	0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0	0	0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		402,000	403,000	800,000		801,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0		0
Pre-tax gains (losses) recognized during the period in income		0	0	0		0
Designated as Hedging Instrument | Cash Flow Hedges | Interest Rate
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0	0	0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0	0	0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	[1]	402,000	403,000	800,000		801,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0		0
Pre-tax gains (losses) recognized during the period in income		0	0	0		0
Other Derivative Instruments
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0	0	0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(249,000)	(1,621,000)	(420,000)		(7,008,000)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0	0	0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	2,749,000		282,000
Pre-tax gains (losses) recognized during the period in income		200,000	(192,000)	(857,000)		(2,803,000)
Other Derivative Instruments | Commodity Trading
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0	0	0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0	0	0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0	0	0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0		0
Pre-tax gains (losses) recognized during the period in income	[2]	200,000	(192,000)	724,000		187,000
Other Derivative Instruments | Natural Gas Commodity
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0	0	0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(249,000)	(1,621,000)	(420,000)		(7,008,000)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0	0	0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	2,749,000	[3]	282,000	[3]
Pre-tax gains (losses) recognized during the period in income	[3]	0	0	(1,581,000)		(2,990,000)
Other Derivative Instruments | Natural Gas Commodity for Electric Generation
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		$ 0	$ 0	$ (1,200,000)		$ 900,000

[1] (a) Amounts are recorded to interest charges.

[2] (b) Amounts are recorded to interest charges. Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue as appropriate.

[3] (c) Certain derivatives are utilized to mitigate natural gas price risk for electric generation and are recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. Amounts for the three and six months ended June 30, 2018 included no settlement gains or losses and $1.2 million of settlement losses, respectively. Amounts for the three and six months ended June 30, 2017 included no settlement gains or losses and $0.9 million of settlement gains, respectively. The remaining derivative settlement gains and losses for the six months ended June 30, 2018 and 2017 relate to natural gas operations and are recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

XML

Fair Value of Financial Assets and Liabilities, Credit Related Contingent Features (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Derivative instruments in a gross liability position	$ 0	$ 0
Collateral posted on derivative instruments	0	0
Collateral posted related to adequate assurance clauses in derivative contracts	$ 0	$ 0

XML

Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands		Jun. 30, 2018		Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Collateral, Obligation to Return Cash, Offset		$ 0		$ 0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		2,700		0
Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		5,340		3,197
Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,716		1,009
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		5,428		7,348
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		2,471		3,468
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		4,452		1,482
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		9,729	[1]	3,564	[2]
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		3,533		1,474
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		9,729	[1]	3,554	[2]
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		919		8
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		0	[1]	10	[2]
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,716		978
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		0	[1]	563	[2]
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,716		978
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[1]	0		563
Fair Value Measured on a Recurring Basis | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		268		2,312
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		(12,474)	[1]	(3,441)	[2]
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		268		1,306
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		(12,474)	[1]	(3,431)	[2]
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability				1,006
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[2]			(10)
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		2,382		799
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[2]			(563)
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		2,382		799
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		0	[1]	(563)	[2]
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		332		528
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		332		528
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0		0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		1		0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		1		0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		333		446
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		333		446
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability				0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		13,731		4,506
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		12,812		4,488
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		919		18
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,715		1,541
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,715		1,541
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		12,409		5,301
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		12,409		4,285
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability				1,016
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		2,382		1,362
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		2,382		1,362
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		118		12
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		118		12
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0		0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0		0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0		0
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		6
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		6
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability				0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0
Fair Value, Measurements, Nonrecurring | Other Current Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		888	[3]	1,715	[4]
Fair Value, Measurements, Nonrecurring | Other Noncurrent Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0	[3]	31	[4]
Fair Value, Measurements, Nonrecurring | Other Current Liabilities | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		5,160	[3]	5,036	[4]
Fair Value, Measurements, Nonrecurring | Other Noncurrent Liabilities | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		89	[3]	2,669	[4]
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		14,181		5,046
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		13,262		5,028
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		919		18
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,716		1,541
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,716		1,541
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		12,742		5,753
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		12,742		4,737
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability				1,016
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		2,382		1,362
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		$ 2,382		$ 1,362

[1] PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at June 30, 2018. At June 30, 2018, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $2.7 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

[2] PSCo nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

[3] During 2006, PSCo qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

[4] During 2006, PSCo qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

XML

Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	$ 100,000		$ 0
Transfers into Level 3	0	$ 0	0	$ 0
Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0

XML

Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 5,300,062	$ 4,608,275
Fair Value
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 5,464,543	$ 5,024,840

XML

Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Income and Expenses [Abstract]
Interest income	$ 508	$ 609	$ 372	$ 984
Other Nonoperating Income	483	1,326	956	4,747
Insurance Policy Expense (Income), Net	(73)	(103)	(150)	(182)
Defined Benefit Plan, Non-service Costs	(119)	(513)	(148)	(1,026)
Other income, net	$ 799	$ 1,319	$ 1,030	$ 4,523

XML

Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Operating revenues		$ 911,866	$ 930,916	$ 1,985,164	$ 2,011,450
Net income		122,296	100,587	256,018	212,133
Affiliate electric revenue		0	0	0	1,000
Affiliate other revenue		1,000	1,000	2,000	2,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		716,271	729,987	1,414,657	1,441,467
Net income		101,956	87,403	181,507	163,547
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		186,740	192,817	550,790	549,009
Net income		20,963	12,835	74,675	47,318
All Other
Segment Reporting Information [Line Items]
Operating revenues		9,010	8,219	20,048	21,229
Net income		(623)	349	(164)	1,268
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1],[2]	911,866	930,916	1,985,164	2,011,450
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]	716,195	729,920	1,414,469	1,441,308
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		186,661	192,777	550,647	548,913
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[2]	9,010	8,219	20,048	21,229
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(155)	(107)	(331)	(255)
Net income		0	0	0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		76	67	188	159
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		79	40	143	96
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0	$ 0	$ 0

[1] Operating revenues include $0 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017.

[2] Operating revenues include $1 million of other affiliate revenue for the three months ended June 30, 2018 and 2017.

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Benefit Plans and Other Postretirement Benefits (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2018 USD ($) Plan	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Other Postretirement Benefits Plan [Member]
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 152	$ 192	$ 304	$ 384
Interest cost		3,748	4,191	7,497	8,382
Expected return on plan assets		(5,674)	(5,476)	(11,349)	(10,952)
Amortization of prior service credit		(1,544)	(1,562)	(3,089)	(3,124)
Amortization of net loss		1,021	961	2,042	1,922
Net periodic benefit cost (credit)		(2,297)	(1,694)	(4,595)	(3,388)
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation		0	0	0	0
Net benefit cost (credit) recognized for financial reporting		(2,297)	(1,694)	(4,595)	(3,388)
Pension Plan [Member]
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		7,271	6,820	14,542	13,640
Interest cost		11,814	12,640	23,628	25,280
Expected return on plan assets		(17,130)	(17,134)	(34,260)	(34,268)
Amortization of prior service credit		(845)	(803)	(1,690)	(1,606)
Amortization of net loss		7,815	7,089	15,630	14,178
Net periodic benefit cost (credit)		8,925	8,612	17,850	17,224
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation		895	426	2,370	1,162
Net benefit cost (credit) recognized for financial reporting		$ 9,820	$ 9,038	$ 20,220	$ 18,386
Total contributions to the pension plans during the period	$ 22,000
Pension Plan [Member] | Xcel Energy Inc.
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Total contributions to the pension plans during the period	$ 150,000
Number of Xcel Energy's pension plans to which contributions were made | Plan	4

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Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 5,828,323
Losses reclassified from net accumulated other comprehensive loss	$ 305	$ 251	607	$ 498
Accumulated other comprehensive loss at end of period	6,129,579		6,129,579
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(26,165)	(22,534)	(26,465)	(22,780)
Losses reclassified from net accumulated other comprehensive loss	303	250	603	496
Net current period other comprehensive loss	303	250	603	496
Accumulated other comprehensive loss at end of period	(25,862)	(22,284)	(25,862)	(22,284)
Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(265)	(219)	(267)	(220)
Losses reclassified from net accumulated other comprehensive loss	2	1	4	2
Net current period other comprehensive loss	2	1	4	2
Accumulated other comprehensive loss at end of period	(263)	(218)	(263)	(218)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(26,430)	(22,753)	(26,732)	(23,000)
Losses reclassified from net accumulated other comprehensive loss	305	251	607	498
Net current period other comprehensive loss	305	251	607	498
Accumulated other comprehensive loss at end of period	$ (26,125)	$ (22,502)	$ (26,125)	$ (22,502)

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Other Comprehensive Income (Reclassification from AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		$ (157,601)	$ (156,998)	$ (330,332)	$ (333,769)
Tax (benefit) expense		35,305	56,411	74,314	121,636
Total amounts reclassified, net of tax		305	251	607	498
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		303	250	603	496
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		402	403	800	801
Tax (benefit) expense		(99)	(153)	(197)	(305)
Total, net of tax		303	250	603	496
Gains and Losses on Cash Flow Hedges | Interest Rate Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	[1]	402	403	800	801
Amortizaton of net loss | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	[2]	2	2	4	4
Defined Benefit and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		2	1	4	2
Defined Benefit and Postretirement Items | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		2	2	4	4
Tax benefit		0	(1)	0	(2)
Total amounts reclassified, net of tax		$ 2	$ 1	$ 4	$ 2

[1] Included in interest charges.

[2] Included in the computation of net periodic pension and postretirement benefit costs. See Note 11 to the consolidated financial statements for details regarding these benefit plans.

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Revenues (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 851,487	$ 891,069	$ 1,862,129	$ 1,919,434
Alternative revenue and other		60,379	39,847	123,035	92,016
Total operating revenues		911,866	930,916	$ 1,985,164	2,011,450
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs				24 months
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[1],[2]	911,866	930,916	$ 1,985,164	2,011,450
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		768,639	799,433	1,675,379	1,730,278
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		329,247	345,953	787,338	808,861
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		427,937	441,769	864,350	896,968
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		11,455	11,711	23,691	24,449
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		36,148	41,579	84,038	85,155
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		13,159	13,180	25,411	27,819
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		33,541	36,877	77,301	76,182
Regulated Electric
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		681,852	701,508	1,343,876	1,383,224
Alternative revenue and other		34,343	28,412	70,593	58,084
Total operating revenues		716,271	729,987	1,414,657	1,441,467
Regulated Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	716,195	729,920	1,414,469	1,441,308
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		617,849	629,912	1,200,900	1,236,718
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		222,687	225,921	450,336	460,355
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		383,707	392,280	726,933	751,977
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		11,455	11,711	23,631	24,386
Regulated Electric | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		36,148	41,579	84,038	85,155
Regulated Electric | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		13,159	13,180	25,411	27,819
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		14,696	16,837	33,527	33,532
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		161,749	182,465	500,452	517,227
Alternative revenue and other		24,912	10,312	50,195	31,686
Total operating revenues		186,740	192,817	550,790	549,009
Regulated Natural Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues		186,661	192,777	550,647	548,913
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		142,904	162,425	456,678	474,577
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		104,000	117,393	331,746	343,303
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		38,904	45,032	124,932	131,274
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
Regulated Natural Gas | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
Regulated Natural Gas | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		18,845	20,040	43,774	42,650
All Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		7,886	7,096	17,801	18,983
Alternative revenue and other		1,124	1,123	2,247	2,246
Total operating revenues		9,010	8,219	20,048	21,229
All Other | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[2]	9,010	8,219	20,048	21,229
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		7,886	7,096	17,801	18,983
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		2,560	2,639	5,256	5,203
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		5,326	4,457	12,485	13,717
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	60	63
All Other | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
All Other | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 0	$ 0	$ 0	$ 0

[1] Operating revenues include $0 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017.

[2] Operating revenues include $1 million of other affiliate revenue for the three months ended June 30, 2018 and 2017.