Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Long-Term Borrowings and Other Financing Instruments
Unamortized discount	$ (4)	$ (3)
Unamortized debt expense	(9)	(8)
Long-term debt, noncurrent	887	808
2021	19
2022	0
2023	0
2024	200
2025	0
NSP Wisconsin [Member]
Long-Term Borrowings and Other Financing Instruments
Long-term Debt, Current Maturities	$ (19)	0
Series Due Nov. 1, 2021 [Member] \| City of La Crosse Resource Recovery Bond
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	6.00%
Debt Instrument, Face Amount	$ 19	19
Series Due June 15, 2024 [Member] \| First Mortgage Bonds
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	3.30%
Debt Instrument, Face Amount	$ 100	100
Series Due June 15, 2024 2 [Member] \| First Mortgage Bonds
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	3.30%
Debt Instrument, Face Amount	$ 100	100
Series Due Sept. 1, 2038 [Member] \| First Mortgage Bonds
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	6.38%
Debt Instrument, Face Amount	$ 200	200
Series Due Oct. 1, 2042 [Member] \| First Mortgage Bonds
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	3.70%
Debt Instrument, Face Amount	$ 100	100
Series Due December 1, 2047 [Member] \| First Mortgage Bonds
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	3.75%
Debt Instrument, Face Amount	$ 100	100
Series Due December 1, 2048 [Member] \| First Mortgage Bonds
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	4.20%
Debt Instrument, Face Amount	$ 200	200
Series Due May 1, 2051 [Member] \| First Mortgage Bonds \| NSP Wisconsin [Member]
Long-Term Borrowings and Other Financing Instruments
Debt Instrument, Interest Rate, Stated Percentage	3.05%
Debt Instrument, Face Amount	$ 100	$ 0

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

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

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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