Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ 942.4
Accumulated other comprehensive loss at end of period		966.6	$ 942.4
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		0.0	(0.1)
Net current period other comprehensive income (loss)		0.0	0.1
Accumulated other comprehensive loss at end of period		0.0	0.0
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of losses, Derivatives	[1]	$ 0.0	$ 0.1

[1] Included in interest charges.