UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net loss	$ (25.3)	$ (1,547.6)	$ (105.0)	$ (1,679.1)
Postretirement plans (net of $0.0 tax provisions in each period)	(11.0)	(2.2)	(22.0)	(4.4)
Foreign currency translation adjustment	(0.2)	6.1	(0.4)	(0.7)
Other comprehensive (loss) income, net of income taxes	(11.2)	3.9	(22.4)	(5.1)
Comprehensive loss	(36.5)	(1,543.7)	(127.4)	(1,684.2)
Less: Net income (loss) attributable to noncontrolling interests	3.3	(3.4)	3.7	(5.2)
Comprehensive loss attributable to common stockholders	$ (39.8)	$ (1,540.3)	$ (131.1)	$ (1,679.0)