Segment and Geographic Information - Reconciliation of Consolidated Income (Loss), Net of Income Taxes to Adjusted EBITDA (Details) - USD ($)	1 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
(Loss) income from continuing operations, net of income taxes		$ (1,859,800,000)	$ (188,300,000)	$ 645,700,000
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
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
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 - equipment		0	(91,100,000)	0
Changes in deferred tax asset valuation allowance and reserves and amortization of basis difference related to equity affiliates		30,900,000	(18,800,000)	(18,300,000)
Interest expense		139,800,000	144,200,000	151,300,000
Interest income		(9,400,000)	(27,000,000.0)	(33,600,000)
Net 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)
Unrealized losses (gains) on economic hedges		29,600,000	(42,200,000)	(18,300,000)
Unrealized (gains) losses on non-coal trading derivative contracts		(7,100,000)	(1,200,000)	700,000
Take-or-pay contract-based intangible recognition		(8,200,000)	(16,600,000)	(26,700,000)
Income tax provision		8,000,000.0	46,000,000.0	18,400,000
Total Adjusted EBITDA		258,800,000	883,000,000.0	1,387,900,000
Insurance recovery		$ 0	125,000,000.0	0
North Goonyella Mine
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Insurance recovery	$ 125,000,000		125,000,000.0
Provision for equipment loss excluded from EBITDA			24,700,000	$ 66,400,000
Business Interruption Losses			$ 33,900,000