Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) - USD ($)	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Deferred Finance Costs, Noncurrent, Net	$ 5,100,000	$ 4,300,000
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met	$ 33,300,000
Minimum calendar year average equity to total capitalization ratio authorized by state commission	52.50%
Calendar year average equity to total capitalization ratio	52.60%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 2,400,000
First Mortgage Bonds \| Series Due June 15, 2024
Debt Instrument [Line Items]
Face Amount		$ 100,000,000
Interest Rate, Stated Percentage	3.30%	3.30%
Maturity Date	Jun. 15, 2024	Jun. 15, 2024
First Mortgage Bonds \| Series Due Oct. 1, 2018
Debt Instrument [Line Items]
Interest Rate, Stated Percentage	5.25%	5.25%
Maturity Date	Oct. 01, 2018	Oct. 01, 2018
Long-term Debt, Maturities, Repayments of Principal in Year Four	$ 150,000,000
NSP-Wisconsin \| First Mortgage Bonds \| Series Due June 15, 2024
Debt Instrument [Line Items]
Face Amount	$ 100,000,000
Interest Rate, Stated Percentage	3.30%
Maturity Date	Jun. 15, 2024

The actual calendar year ratio of average equity to total capitalization.

No definition available.

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- Definition

Maximum annual dividends which can be paid by the entity if its calendar year average equity-to-total capitalization ratio is or falls below the state commission authorized level.

+ References

No definition available.

+ Details

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- Definition

The minimum calendar year average ratio of equity to total capitalization authorized by the state commission to be maintained by the entity in order to pay a maximum stated amount of 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

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

+ Details

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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

+ 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

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

+ 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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- 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 fourth 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 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

Reference 2: 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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