Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	$ 981.3
Balance, end of period	891.5	$ 2,533.3
Foreign Currency Translation Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	1.8
Reclassification from other comprehensive income to earnings	0.0
Current period change	(0.2)
Balance, end of period	1.6
Prior Service Credit Associated with Postretirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	204.0
Reclassification from other comprehensive income to earnings	(11.0)	2.2
Current period change	0.0
Balance, end of period	193.0
Accumulated other comprehensive income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	205.8	31.6
Reclassification from other comprehensive income to earnings	(11.0)
Current period change	(0.2)
Balance, end of period	$ 194.6	$ 22.6