UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenues	$ 679.0	$ 671.0	$ 2,053.7	$ 2,143.9
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)	649.4	550.9	1,843.4	1,886.7
Depreciation, depletion and amortization	77.9	72.2	223.3	266.5
Asset retirement obligation expenses	14.3	14.3	45.3	46.0
Selling and administrative expenses	21.1	27.2	64.2	77.3
Restructuring charges	1.7	8.1	5.9	31.1
Transaction costs related to joint ventures	0.0	6.0	0.0	23.1
Other operating (income) loss:
Net gain on disposals	(25.8)	(2.5)	(28.2)	(10.4)
(Income) loss from equity affiliates	(15.8)	10.6	(11.4)	25.7
Asset impairment	0.0	0.0	0.0	1,418.1
Operating loss	(43.8)	(15.8)	(88.8)	(1,620.2)
Interest expense	45.5	34.9	143.3	102.3
Net gain on early debt extinguishment	16.0	0.0	31.3	0.0
Interest income	(1.4)	(1.6)	(4.2)	(7.1)
Net periodic benefit (credit) costs, excluding service cost	(8.6)	2.8	(26.0)	8.3
Defined Benefit Plan, Net Mark-To-Market Adjustment on Actuarially Determined Liabilities	0.0	13.0	0.0	13.0
Loss from continuing operations before income taxes	(63.3)	(64.9)	(170.6)	(1,736.7)
Income tax (benefit) provision	(3.7)	(0.1)	(10.3)	2.7
Loss from continuing operations, net of income taxes	(59.6)	(64.8)	(160.3)	(1,739.4)
Income (loss) from discontinued operations, net of income taxes	24.3	(2.3)	20.0	(6.8)
Net loss	(35.3)	(67.1)	(140.3)	(1,746.2)
Less: Net income (loss) attributable to noncontrolling interests	8.9	0.1	12.6	(5.1)
Net loss attributable to common stockholders	$ (44.2)	$ (67.2)	$ (152.9)	$ (1,741.1)
Loss from continuing operations:
Basic loss per share	$ (0.60)	$ (0.66)	$ (1.65)	$ (17.76)
Diluted loss per share	(0.60)	(0.66)	(1.65)	(17.76)
Net loss attributable to common stockholders:
Basic loss per share	(0.38)	(0.69)	(1.46)	(17.83)
Diluted loss per share	$ (0.38)	$ (0.69)	$ (1.46)	$ (17.83)
Defined Benefit Plan, Net Mark-To-Market Adjustment on Actuarially Determined Liabilities	$ 0.0	$ 13.0	$ 0.0	$ 13.0
Net gain on early debt extinguishment	16.0	0.0	31.3	0.0
Net periodic benefit (credit) costs, excluding service cost	$ (8.6)	$ 2.8	$ (26.0)	$ 8.3