CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenues	$ 2,881,100,000	$ 4,623,400,000	$ 5,581,800,000
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)	2,524,900,000	3,536,600,000	4,071,400,000
Depreciation, depletion and amortization	346,000,000.0	601,000,000.0	679,000,000.0
Asset retirement obligation expenses	45,700,000	58,400,000	53,000,000.0
Selling and administrative expenses	99,500,000	145,000,000.0	158,100,000
Restructuring charges	37,900,000	24,300,000	1,200,000
Transaction costs related to business combinations and joint ventures	23,100,000	21,600,000	7,400,000
Other operating (income) loss:
Net gain on disposals	(15,200,000)	(2,100,000)	(48,200,000)
Gain on formation of United Wambo Joint Venture	0	(48,100,000)	0
Asset impairment	1,487,400,000	270,200,000	0
Provision for North Goonyella equipment loss	0	83,200,000	66,400,000
North Goonyella insurance recovery	0	(125,000,000.0)	0
Loss (income) from equity affiliates	60,100,000	(3,400,000)	(68,100,000)
Operating profit (loss)	(1,728,300,000)	61,700,000	661,600,000
Interest expense	139,800,000	144,200,000	151,300,000
Interest income	(9,400,000)	(27,000,000.0)	(33,600,000)
Net periodic benefit (credit) costs, excluding service cost	(1,800,000)	19,400,000	18,100,000
Net mark-to-market adjustment on actuarially determined liabilities	(5,100,000)	67,400,000	(125,500,000)
Reorganization items, net	0	0	(12,800,000)
(Loss) income from continuing operations before income taxes	(1,851,800,000)	(142,300,000)	664,100,000
Income tax provision	8,000,000.0	46,000,000.0	18,400,000
(Loss) income from continuing operations, net of income taxes	(1,859,800,000)	(188,300,000)	645,700,000
(Loss) income from discontinued operations, net of income taxes	(14,000,000.0)	3,200,000	18,100,000
Net (loss) income	(1,873,800,000)	(185,100,000)	663,800,000
Less: Series A Convertible Preferred Stock dividends	0	0	102,500,000
Less: Net (loss) income attributable to noncontrolling interests	(3,500,000)	26,200,000	16,900,000
Net (loss) income attributable to common stockholders	$ (1,870,300,000)	$ (211,300,000)	$ 544,400,000
(Loss) income from continuing operations:
Basic (loss) income per share (in dollars per share)	$ (18.99)	$ (2.07)	$ 4.35
Diluted (loss) income per share (in dollars per share)	(18.99)	(2.07)	4.28
Net (loss) income attributable to common stockholders:
Basic (loss) income per share (in dollars per share)	(19.14)	(2.04)	4.50
Diluted (loss) income per share (in dollars per share)	$ (19.14)	$ (2.04)	$ 4.43