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Impact of Derivative Activities on Income and Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]      
Derivative instruments designated as fair value hedges $ 0.0   $ 0.0
Other Derivative Instruments | Electric Commodity      
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]      
Changes in fair value of FTRs pre-tax gains 0.1 $ 6.3  
FTR settlement gains 2.8    
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Interest Rate Contract [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net $ 0.0 $ 0.0