UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018
Cash Flows From Operating Activities
Net (loss) income	$ (67,100,000)	$ (78,100,000)	$ (1,746,200,000)	$ 91,300,000
Loss from discontinued operations, net of income taxes	2,300,000	3,800,000	6,800,000	10,600,000
(Loss) income from continuing operations, net of income taxes	(64,800,000)	(74,300,000)	(1,739,400,000)	101,900,000
Adjustments to reconcile (loss) income from continuing operations, net of income taxes to net cash (used in) provided by operating activities:
Depreciation, depletion and amortization	72,200,000	141,500,000	266,500,000	479,400,000
Noncash interest expense, net			12,000,000.0	11,400,000
Noncash coal inventory revaluation			20,700,000	0
Deferred income taxes			100,000	(400,000)
Noncash share-based compensation			9,900,000	30,200,000
Asset impairment	0	20,000,000.0	1,418,100,000	20,000,000.0
Net gain on disposals	(2,500,000)	(1,100,000)	(10,400,000)	(2,800,000)
Loss from equity affiliates	10,600,000	20,700,000	25,700,000	7,500,000
Provision for North Goonyella equipment loss	0	0	0	24,700,000	$ 66,400,000
Foreign currency option contracts			(5,200,000)	3,500,000
Changes in current assets and liabilities:
Accounts receivable			136,600,000	118,900,000
Inventories			(8,800,000)	(15,100,000)
Other current assets			300,000	(27,300,000)
Accounts payable and accrued expenses			(136,300,000)	(115,100,000)
Asset retirement obligations			12,200,000	9,100,000
Workers’ compensation obligations			(1,300,000)	500,000
Postretirement benefit obligations			(6,100,000)	(37,800,000)
Pension obligations			300,000	(16,900,000)
Other, net			(4,600,000)	(13,900,000)
Net cash (used in) provided by continuing operations			(9,700,000)	577,800,000
Net cash used in discontinued operations			(22,400,000)	(25,200,000)
Net cash (used in) provided by operating activities			(32,100,000)	552,600,000
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development			(131,900,000)	(182,800,000)
Changes in accrued expenses related to capital expenditures			(14,900,000)	(5,600,000)
Insurance proceeds attributable to North Goonyella equipment losses			0	23,200,000
Proceeds from disposal of assets, net of receivables			15,400,000	27,600,000
Amount attributable to acquisition of Shoal Creek Mine			0	(2,400,000)
Contributions to joint ventures			(275,200,000)	(326,400,000)
Distributions from joint ventures			271,000,000.0	316,700,000
Advances to related parties			(23,100,000)	(12,500,000)
Cash receipts from Middlemount Coal Pty Ltd			0	14,700,000
Other, net			(700,000)	(100,000)
Net cash used in investing activities			(159,400,000)	(147,600,000)
Cash Flows From Financing Activities
Proceeds from long-term debt			360,000,000.0	0
Repayments of long-term debt			(81,000,000.0)	(23,900,000)
Payment of debt issuance and other deferred financing costs			0	(6,400,000)
Common stock repurchases			0	(300,200,000)
Repurchase of employee common stock relinquished for tax withholding			(1,600,000)	(12,300,000)
Dividends paid			0	(243,900,000)
Distributions to noncontrolling interests			(3,500,000)	(23,400,000)
Other, net			0	100,000
Net cash provided by (used in) financing activities			273,900,000	(610,000,000.0)
Net change in cash, cash equivalents and restricted cash			82,400,000	(205,000,000.0)
Cash, cash equivalents and restricted cash at beginning of period			732,200,000	1,017,400,000
Cash, cash equivalents and restricted cash at end of period	$ 814,600,000	$ 812,400,000	$ 814,600,000	$ 812,400,000	$ 1,017,400,000