Impact of Derivative Activities on Income and Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 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	$ 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	2.5		5.0	4.7
FTR settlement gains	2.3	$ 1.7	3.6	$ 1.5
Designated as Hedging Instrument [Member] | Interest Rate Contract [Member] | Cash Flow Hedging [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.0		$ 0.0