Consolidated Statements of Operations - USD ($) $ in Millions		3 Months Ended									9 Months Ended	12 Months Ended
	Nov. 28, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Revenues		$ 1,117.4	$ 1,106.4	$ 1,149.0	$ 1,250.6	$ 1,397.1	$ 1,412.6	$ 1,309.4	$ 1,462.7	$ 1,326.2	$ 4,252.6	$ 4,623.4	$ 5,581.8
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)										950.2	3,045.1	3,536.6	4,071.4
Depreciation, depletion and amortization										119.9	521.6	601.0	679.0
Asset retirement obligation expenses										14.6	41.2	58.4	53.0
Selling and administrative expenses										36.3	106.3	145.0	158.1
Restructuring charges	$ 3.0	23.0								0.0	7.6	24.3	1.2
Transaction costs related to business combinations and joint ventures		11.8	8.2			4.9				0.0	0.0	21.6	7.4
Other operating (income) loss:
Net gain on disposals							(20.7)		(30.6)	(22.8)	(84.0)	(2.1)	(48.2)
Gain on formation of United Wambo JV		(48.1)								0.0	0.0	(48.1)	0.0
Asset impairment		250.2	20.0							30.5	0.0	270.2	0.0
Provision for North Goonyella equipment loss		58.5			24.7	17.1	49.3			0.0	0.0	83.2	66.4
North Goonyella insurance recovery					(125.0)					0.0	0.0	(125.0)	0.0
Income from equity affiliates		(10.9)	(20.7)	(9.7)			(17.2)	(25.2)	(22.0)	(15.0)	(49.0)	(3.4)	(68.1)
Operating profit		(165.5)	(36.8)	79.5	184.5	126.8	130.3	165.3	239.2	212.5	663.8	61.7	661.6
Interest expense		36.8	35.4	36.0	35.8	36.5	38.2	38.3	36.3	32.9	119.7	144.0	149.3
Loss on early debt extinguishment										0.0	20.9	0.2	2.0
Interest income		(4.5)	(7.0)	(7.2)	(8.3)	(9.3)	(10.1)	(7.0)	(7.2)	(2.7)	(5.6)	(27.0)	(33.6)
Net periodic benefit costs, excluding service cost										14.4	21.9	19.4	18.1
Net mark-to-market adjustment on actuarially determined liabilities		67.4				125.5				0.0	(45.2)	67.4	(125.5)
Reorganization items, net									12.8	627.2	0.0	0.0	(12.8)
(Loss) income from continuing operations before income taxes										(459.3)	552.1	(142.3)	664.1
Income tax provision (benefit)										(263.8)	(161.0)	46.0	18.4
(Loss) income from continuing operations, net of income taxes		(290.2)	(74.3)	42.9	133.3	233.5	83.9	120.0	208.3	(195.5)	713.1	(188.3)	645.7
Income (loss) from discontinued operations, net of income taxes										(16.2)	(19.8)	3.2	18.1
Net (loss) income		(276.4)	(78.1)	39.5	129.9	260.6	79.8	116.4	207.0	(211.7)	693.3	(185.1)	663.8
Less: Series A Convertible Preferred Stock dividends										0.0	179.5	0.0	102.5
Less: Net income attributable to noncontrolling interests										4.8	15.2	26.2	16.9
Net (loss) income attributable to common stockholders		$ (289.8)	$ (82.8)	$ 37.1	$ 124.2	$ 252.6	$ 71.5	$ 113.7	$ 106.6	$ (216.5)	$ 498.6	$ (211.3)	$ 544.4
(Loss) income from continuing operations:
Basic (loss) income per share		$ (3.12)	$ (0.77)	$ 0.38	$ 1.18	$ 1.99	$ 0.64	$ 0.94	$ 0.84	$ (10.93)	$ 3.85	$ (2.07)	$ 4.35
Diluted (loss) income per share		$ (3.12)	$ (0.77)	$ 0.37	$ 1.15	$ 1.97	$ 0.63	$ 0.93	$ 0.83	(10.93)	3.81	(2.07)	4.28
Net (loss) income attributable to common stockholders:
Basic (loss) income per share										(11.81)	3.70	(2.04)	4.50
Diluted (loss) income per share										$ (11.81)	$ 3.67	$ (2.04)	$ 4.43