Segment and Geographic Information - Segment Results (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 2,881.1	$ 4,623.4	$ 5,581.8
Adjusted EBITDA	258.8	883.0	1,387.9
Additions to property, plant, equipment and mine development	191.4	285.4	301.0
Federal Coal Lease Expenditures			0.5
Loss (income) from equity affiliates	60.1	(3.4)	(68.1)
Seaborne Thermal Mining
Segment Reporting Information [Line Items]
Revenues	711.8	971.7	1,099.2
Adjusted EBITDA	163.2	329.4	452.0
Additions to property, plant, equipment and mine development	100.7	42.1	66.6
Federal Coal Lease Expenditures			0.0
Loss (income) from equity affiliates	0.0	0.0	0.0
Seaborne Metallurgical Mining
Segment Reporting Information [Line Items]
Revenues	486.5	1,033.1	1,553.0
Adjusted EBITDA	(130.2)	140.2	441.4
Additions to property, plant, equipment and mine development	50.8	143.4	88.7
Federal Coal Lease Expenditures			0.0
Loss (income) from equity affiliates	0.0	0.0	0.0
Powder River Basin Mining
Segment Reporting Information [Line Items]
Revenues	991.1	1,228.7	1,424.8
Adjusted EBITDA	194.8	221.2	284.5
Additions to property, plant, equipment and mine development	13.2	42.8	81.0
Federal Coal Lease Expenditures			0.0
Loss (income) from equity affiliates	0.0	0.0	0.0
Other U.S. Thermal Mining
Segment Reporting Information [Line Items]
Revenues	707.3	1,309.4	1,393.0
Adjusted EBITDA	168.4	361.4	290.6
Additions to property, plant, equipment and mine development	23.3	54.0	60.5
Federal Coal Lease Expenditures			0.5
Loss (income) from equity affiliates	0.0	0.0	0.0
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	(15.6)	80.5	111.8
Adjusted EBITDA	(137.4)	(169.2)	(80.6)
Additions to property, plant, equipment and mine development	3.4	3.1	4.2
Federal Coal Lease Expenditures			0.0
Loss (income) from equity affiliates	$ 60.1	$ (3.4)	$ (68.1)

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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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