Segment Information - Reconciliation to Adjusted EBITDA (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended	18 Months Ended
	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2019
Segment Reporting Information [Line Items]
(Loss) income from continuing operations, net of income taxes		$ (64,800,000)	$ (74,300,000)	$ (1,739,400,000)	$ 101,900,000
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
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
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	$ 66,400,000
North Goonyella insurance recovery - equipment		0	0	0	(91,100,000)
Changes in deferred tax asset valuation allowance and reserves and amortization of basis difference related to equity affiliates		(500,000)	0	(1,600,000)	300,000
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 mark-to-market adjustment on actuarially determined liabilities		13,000,000.0	0	13,000,000.0	0
Unrealized losses (gains) on economic hedges		16,100,000	18,000,000.0	11,300,000	(44,200,000)
Unrealized gains on non-coal trading derivative contracts		(700,000)	(300,000)	(3,600,000)	(200,000)
Take-or-pay contract-based intangible recognition		(1,500,000)	(2,700,000)	(6,800,000)	(13,900,000)
Income tax (benefit) provision		(100,000)	4,200,000	2,700,000	26,000,000.0
Adjusted EBITDA		95,400,000	159,200,000	155,600,000	643,300,000
North Goonyella insurance recovery		$ 0	$ 0	0	125,000,000.0
Business Interruption Losses				$ 33,900,000
North Goonyella Mine
Segment Reporting Information [Line Items]
Provision for North Goonyella equipment loss					24,700,000		$ 149,600,000
North Goonyella insurance recovery	$ 125,000,000				$ 125,000,000.0