Other Comprehensive Income (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ 967.0
Accumulated other comprehensive loss at end of period	1,068.0
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of losses, Derivatives	$ 0.0	[1]

[1] Included in interest charges.