Segment Information - Segment Results (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reportable segment results
Revenues	$ 651.3	$ 846.2
Adjusted EBITDA	61.1	36.8
Seaborne Thermal Mining
Reportable segment results
Revenues	176.4	201.1
Adjusted EBITDA	28.5	55.1
Seaborne Metallurgical Mining
Reportable segment results
Revenues	87.5	193.2
Adjusted EBITDA	(22.4)	(32.7)
Powder River Basin Mining
Reportable segment results
Revenues	228.4	266.6
Adjusted EBITDA	30.1	25.4
Other U.S. Thermal Mining
Reportable segment results
Revenues	149.3	192.3
Adjusted EBITDA	36.2	38.5
Corporate and Other (1)
Reportable segment results
Revenues	9.7	(7.0)
Adjusted EBITDA	$ (11.3)	$ (49.5)