Unaudited Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Cash Flows From Operating Activities
Net income	$ (78.1)		$ 79.8	$ 91.3	$ 403.2
Loss from discontinued operations, net of income taxes	3.8		4.1	10.6	9.0
Income from continuing operations, net of income taxes	(74.3)		83.9	101.9	412.2
Adjustments to reconcile income from continuing operations, net of income taxes to net cash provided by operating activities:
Depreciation, depletion and amortization	141.5		169.6	479.4	503.1
Noncash interest expense, net				11.4	11.3
Deferred income taxes				(0.4)	17.5
Noncash share-based compensation				30.2	25.6
Asset impairment	20.0		0.0	20.0	0.0
Net gain on disposals	(1.1)		(20.8)	(2.8)	(49.8)
Loss (income) from equity affiliates	20.7		(17.2)	7.5	(64.4)
Provision for North Goonyella equipment loss	0.0		49.3	24.7	49.3		$ 66.4
Foreign currency option contracts				3.5	7.9
Noncash reorganization items, net				0.0	(12.8)
Changes in current assets and liabilities:
Accounts receivable				118.9	177.3
Inventories				(15.1)	14.4
Other current assets				(27.3)	(36.2)
Accounts payable and accrued expenses				(115.1)	(39.0)
Collateral arrangements				0.0	323.1
Asset retirement obligations				9.1	9.5
Workers’ compensation obligations				0.5	(0.4)
Postretirement benefit obligations				(37.8)	(6.6)
Pension obligations				(16.9)	(68.8)
Other, net				(13.9)	10.6
Net cash provided by continuing operations				577.8	1,283.8
Net cash used in discontinued operations				(25.2)	(23.0)
Net cash provided by operating activities				552.6	1,260.8
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development				(182.8)	(186.5)
Changes in accrued expenses related to capital expenditures				(5.6)	(7.0)
Federal coal lease expenditures				0.0	(0.5)
Insurance proceeds attributable to North Goonyella equipment losses				23.2	0.0
Proceeds from disposal of assets, net of receivables				27.6	69.0
Amount attributable to acquisition of Shoal Creek Mine				(2.4)	0.0
Contributions to joint ventures				(326.4)	(358.2)
Distributions from joint ventures				316.7	355.0
Advances to related parties				(12.5)	(5.6)
Cash receipts from Middlemount Coal Pty Ltd				14.7	81.1
Investment in equity securities				0.0	(10.0)
Other, net				(0.1)	(2.8)
Net cash used in investing activities				(147.6)	(65.5)
Cash Flows From Financing Activities
Repayments of long-term debt				(23.9)	(73.0)
Payment of debt issuance and other deferred financing costs				(6.4)	(21.2)
Common stock repurchases				(300.2)	(699.6)
Repurchase of employee common stock relinquished for tax withholding				(12.3)	(14.5)
Dividends paid				(243.9)	(44.6)
Distributions to noncontrolling interests				(23.4)	(10.3)
Other, net				0.1	0.1
Net cash used in financing activities				(610.0)	(863.1)
Net change in cash, cash equivalents and restricted cash				(205.0)	332.2
Cash, cash equivalents and restricted cash at beginning of period		$ 1,402.4		1,017.4	1,070.2	$ 1,402.4	1,070.2
Cash, cash equivalents and restricted cash at end of period	812.4	1,017.4	1,402.4	812.4	1,402.4	812.4	1,017.4
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents								$ 759.1	$ 981.9
Restricted cash included in “Investments and other assets”								53.3	35.5
Cash, cash equivalents and restricted cash	$ 812.4	$ 1,402.4	$ 1,402.4	$ 1,017.4	$ 1,070.2	$ 1,402.4	$ 1,070.2	$ 812.4	$ 1,017.4