UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) $ in Millions	Total	Common Stock	Additional paid-in capital	Treasury stock	(Accumulated deficit) retained earnings	Accumulated other comprehensive income	Noncontrolling interests
Balance, beginning of period at Dec. 31, 2019		$ 1.4	$ 3,351.1	$ (1,367.3)	$ 597.0	$ 31.6	$ 58.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation for equity-classified awards			6.1
Common stock issued in exchange for debt retirement			0.0
Common stock issuances, net of costs			0.0
Net loss	$ (1,679.1)				(1,673.9)		(5.2)
Repurchase of employee common stock relinquished for tax withholding	1.6			(1.6)
Postretirement plans (net of $0.0 tax provisions in each period)	(4.4)					(4.4)
Foreign currency translation adjustment	(0.7)					(0.7)
Distributions to noncontrolling interests							(3.5)
Common stock issuances, net of costs		0.0
Balance, end of period at Jun. 30, 2020	989.3	1.4	3,357.2	(1,368.9)	(1,076.9)	26.5	50.0
Balance, beginning of period at Mar. 31, 2020		1.4	3,353.3	(1,368.1)	467.3	22.6	56.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation for equity-classified awards			3.9
Common stock issued in exchange for debt retirement			0.0
Common stock issuances, net of costs			0.0
Net loss	(1,547.6)				(1,544.2)		(3.4)
Repurchase of employee common stock relinquished for tax withholding				(0.8)
Postretirement plans (net of $0.0 tax provisions in each period)	(2.2)					(2.2)
Foreign currency translation adjustment	6.1					6.1
Distributions to noncontrolling interests							(3.4)
Common stock issuances, net of costs		0.0
Balance, end of period at Jun. 30, 2020	989.3	1.4	3,357.2	(1,368.9)	(1,076.9)	26.5	50.0
Balance, beginning of period at Dec. 31, 2020	981.3	1.4	3,364.6	(1,368.9)	(1,273.3)	205.8	51.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation for equity-classified awards			3.9
Common stock issued in exchange for debt retirement			30.5
Common stock issuances, net of costs			64.8
Net loss	(105.0)				(108.7)		3.7
Repurchase of employee common stock relinquished for tax withholding	1.3			(1.3)
Postretirement plans (net of $0.0 tax provisions in each period)	(22.0)					(22.0)
Foreign currency translation adjustment	(0.4)					(0.4)
Distributions to noncontrolling interests							(0.1)
Common stock issuances, net of costs		0.1
Balance, end of period at Jun. 30, 2021	951.8	1.5	3,463.8	(1,370.2)	(1,382.0)	183.4	55.3
Balance, beginning of period at Mar. 31, 2021		1.4	3,366.4	(1,369.5)	(1,353.4)	194.6	52.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation for equity-classified awards			2.1
Common stock issued in exchange for debt retirement			30.5
Common stock issuances, net of costs			64.8
Net loss	(25.3)				(28.6)		3.3
Repurchase of employee common stock relinquished for tax withholding				(0.7)
Postretirement plans (net of $0.0 tax provisions in each period)	(11.0)					(11.0)
Foreign currency translation adjustment	(0.2)					(0.2)
Common stock issuances, net of costs		0.1
Balance, end of period at Jun. 30, 2021	$ 951.8	$ 1.5	$ 3,463.8	$ (1,370.2)	$ (1,382.0)	$ 183.4	$ 55.3