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Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Deferred Finance Costs, Noncurrent, Net $ 4.7 $ 5.1
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met $ 53.1  
Minimum calendar year average equity to total capitalization ratio authorized by state commission 52.50%  
Calendar year average equity to total capitalization ratio 53.60%  
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions $ 33.6  
First Mortgage Bonds | Series Due June 15, 2024    
Debt Instrument [Line Items]    
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 Two $ 150.0  
City of La Crosse Resource Recovery Bond | Series Due Nov. 1, 2021    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage 6.00% 6.00%
Maturity Date Nov. 01, 2021 Nov. 01, 2021
Long-term Debt, Maturities, Repayments of Principal in Year Five [1] $ 18.6  
NSP-Wisconsin | First Mortgage Bonds | Series Due June 15, 2024    
Debt Instrument [Line Items]    
Face Amount   $ 100.0
Interest Rate, Stated Percentage   3.30%
Maturity Date   Jun. 15, 2024
[1] Resource recovery financing