Segment Information - Segment Results (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reportable segment results
Revenues	$ 723.4	$ 626.7	$ 1,374.7	$ 1,472.9
Adjusted EBITDA	122.1	23.4	183.2	60.2
Seaborne Thermal Mining
Reportable segment results
Revenues	194.1	162.0	370.5	363.1
Adjusted EBITDA	71.4	27.7	99.9	82.8
Seaborne Metallurgical Mining
Reportable segment results
Revenues	121.0	91.6	208.5	284.8
Adjusted EBITDA	(26.4)	(36.1)	(48.8)	(68.8)
Powder River Basin Mining
Reportable segment results
Revenues	248.6	205.8	477.0	472.4
Adjusted EBITDA	45.5	39.3	75.6	64.7
Other U.S. Thermal Mining
Reportable segment results
Revenues	162.1	152.0	311.4	344.3
Adjusted EBITDA	44.3	32.9	80.5	71.4
Corporate and Other (1)
Reportable segment results
Revenues	(2.4)	15.3	7.3	8.3
Adjusted EBITDA	$ (12.7)	$ (40.4)	$ (24.0)	$ (89.9)