Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Deferred Finance Costs, Noncurrent, Net	$ 14.5	$ 12.1
Equity to total capitalization ratio (excluding short-term debt), low end of range	45.00%
Equity to total capitalization ratio (excluding short-term debt), high end of range	55.00%
Equity to total capitalization ratio (excluding short-term debt)	54.10%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 487.0
First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2046
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.40%	3.40%
Maturity Date	Aug. 15, 2046	Aug. 15, 2046
First Mortgage Bonds, Series due: \| Series Due June 15, 2024
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.30%	3.30%
Maturity Date	Jun. 15, 2024	Jun. 15, 2024
Unsecured Debt [Member] \| Senior G Due Dec. 1, 2018
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	8.75%	8.75%
Maturity Date	Dec. 01, 2018	Dec. 01, 2018
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 250.0
SPS \| First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2046
Debt Instrument [Line Items]
Face Amount	$ 300.0
Interest Rate, Stated Percentage (in hundredths)	3.40%
Maturity Date	Aug. 15, 2046
SPS \| First Mortgage Bonds, Series due: \| Series Due June 15, 2024
Debt Instrument [Line Items]
Face Amount		$ 200.0
Interest Rate, Stated Percentage (in hundredths)		3.30%
Maturity Date		Jun. 15, 2024

The actual ratio of equity to total capitalization, excluding short-term debt.

No definition available.

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- Definition

The high end of the range of the ratio of equity to total capitalization, excluding short-term debt, required to be maintained by the entity in order to pay dividends.

+ References

No definition available.

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- Definition

The low end of the range of the ratio of equity to total capitalization, excluding short-term debt, required to be maintained by the entity in order to pay dividends.

+ References

No definition available.

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- Definition

Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions

+ References

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

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