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Borrowings and Other Financing Instruments (Details) - USD ($)
Sep. 30, 2020
May 26, 2020
Dec. 31, 2019
Short-term Debt [Line Items]      
Clearwater Borrowings $ 0   $ 0
Payable to Affiliates      
Short-term Debt [Line Items]      
Notes Payable, Related Parties, Current $ 0   $ 0
Series Due May 1, 2051 [Domain] | Bonds [Member]      
Short-term Debt [Line Items]      
Debt Instrument, Face Amount   $ 100,000,000  
Series Due May 1, 2051 [Domain] | Bonds [Member] | NSP-Wisconsin [Member]      
Debt Disclosure [Abstract]      
Debt Instrument, Interest Rate, Stated Percentage   305.00%  
Short-term Debt [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage   305.00%