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Commitments and Contingencies, Fuel Contracts (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Fuel Contracts [Abstract]  
Minimum annual tolerance band percentage for future rate recovery or refund of fuel costs (in hundredths) 2.00%nspw_MinimumAnnualToleranceBandPercentageForFutureRateRecoveryOrRefundOfFuelCosts
Coal  
Fuel Contracts [Abstract]  
2015 6.6us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_CoalMember
2016 0.8us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnSecondAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_CoalMember
2017 0.9us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnThirdAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_CoalMember
2018 0.8us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFourthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_CoalMember
2019 0.8us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFifthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_CoalMember
Thereafter 3.3us-gaap_UnrecordedUnconditionalPurchaseObligationDueAfterFiveYears
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_CoalMember
Total 13.2us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_CoalMember
[1]
Natural Gas Supply  
Fuel Contracts [Abstract]  
2015 12.4us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasSupplyMember
2016 0.3us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnSecondAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasSupplyMember
2017 0.2us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnThirdAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasSupplyMember
2018 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFourthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasSupplyMember
2019 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFifthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasSupplyMember
Thereafter 0us-gaap_UnrecordedUnconditionalPurchaseObligationDueAfterFiveYears
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasSupplyMember
Total 12.9us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasSupplyMember
[1]
Natural Gas Storage and Transportation  
Fuel Contracts [Abstract]  
2015 13.2us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasStorageAndTransportationMember
2016 13.1us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnSecondAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasStorageAndTransportationMember
2017 10.4us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnThirdAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasStorageAndTransportationMember
2018 4.7us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFourthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasStorageAndTransportationMember
2019 3.1us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFifthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasStorageAndTransportationMember
Thereafter 13.5us-gaap_UnrecordedUnconditionalPurchaseObligationDueAfterFiveYears
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasStorageAndTransportationMember
Total 58.0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nspw_NaturalGasStorageAndTransportationMember
[1]
Minimum  
Unrecorded Unconditional Purchase Obligation [Line Items]  
Fuel Contract Expiration Date 2015
Maximum  
Unrecorded Unconditional Purchase Obligation [Line Items]  
Fuel Contract Expiration Date 2029
[1] Excludes additional amounts allocated to NSP-Wisconsin through intercompany charges.