UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues	$ 671,000,000.0	$ 1,106,400,000	$ 2,143,900,000	$ 3,506,000,000.0
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)	550,900,000	905,500,000	1,886,700,000	2,711,500,000
Depreciation, depletion and amortization	72,200,000	141,500,000	266,500,000	479,400,000
Asset retirement obligation expenses	14,300,000	15,500,000	46,000,000.0	44,600,000
Selling and administrative expenses	27,200,000	32,200,000	77,300,000	107,800,000
Restructuring charges	8,100,000	700,000	31,100,000	1,300,000
Transaction costs related to joint ventures	6,000,000.0	8,200,000	23,100,000	9,800,000
Other operating loss (income):
Net gain on disposals	(2,500,000)	(1,100,000)	(10,400,000)	(2,800,000)
Asset impairment	0	20,000,000.0	1,418,100,000	20,000,000.0
Provision for North Goonyella equipment loss	0	0	0	24,700,000
North Goonyella insurance recovery	0	0	0	(125,000,000.0)
Loss from equity affiliates	10,600,000	20,700,000	25,700,000	7,500,000
Operating (loss) profit	(15,800,000)	(36,800,000)	(1,620,200,000)	227,200,000
Interest expense	34,900,000	35,400,000	102,300,000	107,200,000
Interest income	(1,600,000)	(7,000,000.0)	(7,100,000)	(22,500,000)
Net periodic benefit costs, excluding service cost	2,800,000	4,900,000	8,300,000	14,600,000
Net mark-to-market adjustment on actuarially determined liabilities	13,000,000.0	0	13,000,000.0	0
(Loss) income from continuing operations before income taxes	(64,900,000)	(70,100,000)	(1,736,700,000)	127,900,000
Income tax (benefit) provision	(100,000)	4,200,000	2,700,000	26,000,000.0
(Loss) income from continuing operations, net of income taxes	(64,800,000)	(74,300,000)	(1,739,400,000)	101,900,000
Loss from discontinued operations, net of income taxes	(2,300,000)	(3,800,000)	(6,800,000)	(10,600,000)
Net (loss) income	(67,100,000)	(78,100,000)	(1,746,200,000)	91,300,000
Less: Net income (loss) attributable to noncontrolling interests	100,000	4,700,000	(5,100,000)	12,800,000
Net (loss) income attributable to common stockholders	$ (67,200,000)	$ (82,800,000)	$ (1,741,100,000)	$ 78,500,000
(Loss) income from continuing operations:
Basic (loss) income per share (in dollars per share)	$ (0.66)	$ (0.77)	$ (17.76)	$ 0.84
Diluted (loss) income per share (in dollars per share)	(0.66)	(0.77)	(17.76)	0.83
Net (loss) income attributable to common stockholders:
Basic (loss) income per share (in dollars per share)	(0.69)	(0.81)	(17.83)	0.74
Diluted (loss) income per share (in dollars per share)	$ (0.69)	$ (0.81)	$ (17.83)	$ 0.73