CONSOLIDATED STATEMENTS OF COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 371.4	$ (1,873.8)	$ (185.1)
Postretirement plans (net of $0.0 tax provisions in each period)	93.1	168.1	(8.7)
Foreign currency translation adjustment	(1.0)	6.1	0.2
Other comprehensive income (loss), net of income taxes	92.1	174.2	(8.5)
Comprehensive income (loss)	463.5	(1,699.6)	(193.6)
Less: Net income (loss) attributable to noncontrolling interests	11.3	(3.5)	26.2
Comprehensive income (loss) attributable to common stockholders	$ 452.2	$ (1,696.1)	$ (219.8)