Segment and Geographic Information - Segment Results (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 3,318.3	$ 2,881.1	$ 4,623.4
Adjusted EBITDA	916.7	258.8	883.0
Additions to property, plant, equipment and mine development	183.1	191.4	285.4
(Income) loss from equity affiliates	(82.1)	60.1	(3.4)
Seaborne Thermal Mining
Segment Reporting Information [Line Items]
Revenues	934.0	711.8	971.7
Adjusted EBITDA	353.1	163.2	329.4
Additions to property, plant, equipment and mine development	88.6	100.7	42.1
(Income) loss from equity affiliates	0.0	0.0	0.0
Seaborne Metallurgical Mining
Segment Reporting Information [Line Items]
Revenues	727.7	486.5	1,033.1
Adjusted EBITDA	178.2	(130.2)	140.2
Additions to property, plant, equipment and mine development	25.1	50.8	143.4
(Income) loss from equity affiliates	0.0	0.0	0.0
Powder River Basin Mining
Segment Reporting Information [Line Items]
Revenues	971.2	991.1	1,228.7
Adjusted EBITDA	134.9	194.8	221.2
Additions to property, plant, equipment and mine development	41.4	13.2	42.8
(Income) loss from equity affiliates	0.0	0.0	0.0
Other U.S. Thermal Mining
Segment Reporting Information [Line Items]
Revenues	689.1	707.3	1,309.4
Adjusted EBITDA	164.2	168.4	361.4
Additions to property, plant, equipment and mine development	24.2	23.3	54.0
(Income) loss from equity affiliates	0.0	0.0	0.0
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	(3.7)	(15.6)	80.5
Adjusted EBITDA	86.3	(137.4)	(169.2)
Additions to property, plant, equipment and mine development	3.8	3.4	3.1
(Income) loss from equity affiliates	$ (82.1)	$ 60.1	$ (3.4)