Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - USD ($)	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Deferred Finance Costs, Noncurrent, Net	$ 12,100,000	$ 10,900,000
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)	53.80%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 438,000,000
First Mortgage Bonds \| Series Due June 15, 2024
Debt Instrument [Line Items]
Debt instrument, Interest Rate, Stated Percentage	3.30%	3.30%
Debt Instrument, Maturity Date	Jun. 15, 2024	Jun. 15, 2024
Principal outstanding	$ 350,000,000	$ 150,000,000
First Mortgage Bonds \| Series No. 3 Due June 15, 2024
Debt Instrument [Line Items]
Debt instrument, Face Amount		$ 150,000,000
Debt instrument, Interest Rate, Stated Percentage		3.30%
Debt Instrument, Maturity Date		Jun. 15, 2024
First Mortgage Bonds \| Issued Collateral on Series G Senior Note Due Dec. 1, 2018
Debt Instrument [Line Items]
Debt instrument, Face Amount		$ 250,000,000
Debt instrument, Interest Rate, Stated Percentage		8.75%
Debt Instrument, Maturity Date		Dec. 01, 2018
Unsecured Debt [Member] \| Senior G Due Dec. 1, 2018
Debt Instrument [Line Items]
Debt instrument, Interest Rate, Stated Percentage	8.75%	8.75%
Debt Instrument, Maturity Date	Dec. 01, 2018	Dec. 01, 2018
Principal outstanding	$ 250,000,000	$ 250,000,000
Unsecured Debt [Member] \| Senior E Due Oct. 1, 2016
Debt Instrument [Line Items]
Debt instrument, Interest Rate, Stated Percentage	5.60%	5.60%
Debt Instrument, Maturity Date	Oct. 01, 2016	Oct. 01, 2016
Principal outstanding	$ 200,000,000	$ 200,000,000
SPS [Member] \| First Mortgage Bonds \| Series Due June 15, 2024
Debt Instrument [Line Items]
Debt instrument, Face Amount	$ 200,000,000
Debt instrument, Interest Rate, Stated Percentage	3.30%
SPS [Member] \| First Mortgage Bonds \| Issued Collateral on Series G Senior Note Due Dec. 1, 2018
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Dec. 01, 2018
SPS [Member] \| Unsecured Debt [Member] \| Senior G Due Dec. 1, 2018
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Dec. 01, 2018

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.

+ Details

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- Definition

Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions

+ References

No definition available.

+ Details

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- Definition

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

+ 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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20
-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 16
-Article 9

+ Details

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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=34725769&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=6451184&loc=d3e28551-108399

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- Definition

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

+ References

+ Details

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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

+ Details

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- Definition

Net amount of long-term deferred finance costs capitalized at the end of the reporting period.

+ 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=6451184&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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