Segment Information - Segment Results (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reportable segment results
Revenues	$ 671.0	$ 1,106.4	$ 2,143.9	$ 3,506.0
Adjusted EBITDA	95.4	159.2	155.6	643.3
Seaborne Thermal Mining
Reportable segment results
Revenues	163.0	249.5	526.1	720.7
Adjusted EBITDA	35.3	76.8	118.1	245.9
Seaborne Metallurgical Mining
Reportable segment results
Revenues	78.8	216.3	363.6	831.7
Adjusted EBITDA	(27.3)	(16.2)	(96.1)	127.0
Powder River Basin Mining
Reportable segment results
Revenues	264.8	333.6	737.2	903.5
Adjusted EBITDA	78.3	70.7	143.0	147.3
Other U.S. Thermal Mining
Reportable segment results
Revenues	179.8	326.4	524.1	970.8
Adjusted EBITDA	51.6	82.3	123.0	241.3
Corporate and Other
Reportable segment results
Revenues	(15.4)	(19.4)	(7.1)	79.3
Adjusted EBITDA	$ (42.5)	$ (54.4)	$ (132.4)	$ (118.2)