Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	$ 1,820.8
Balance, end of period	1,896.9	$ 891.5
Postretirement plans (net of $0.0 tax provisions in each period)		11.0
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	0.8
Reclassification from other comprehensive income to earnings	0.0
Current period change	1.9
Balance, end of period	2.7
Prior Service Credit Associated with Postretirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	297.1
Reclassification from other comprehensive income to earnings	(13.4)
Current period change	0.0
Balance, end of period	283.7
Total Accumulated Other Comprehensive Income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	297.9	205.8
Reclassification from other comprehensive income to earnings	(13.4)
Current period change	1.9
Balance, end of period	$ 286.4	$ 194.6