Segment Information - Segment Results (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reportable segment results
Revenues	$ 679.0	$ 671.0	$ 2,053.7	$ 2,143.9
Adjusted EBITDA	289.1	95.4	472.3	155.6
Seaborne Thermal Mining
Reportable segment results
Revenues	260.7	163.0	631.2	526.1
Adjusted EBITDA	104.4	35.3	204.3	118.1
Seaborne Metallurgical Mining
Reportable segment results
Revenues	179.5	78.8	388.0	363.6
Adjusted EBITDA	57.4	(27.3)	8.6	(96.1)
Powder River Basin Mining
Reportable segment results
Revenues	247.1	264.8	724.1	737.2
Adjusted EBITDA	37.0	78.3	112.6	143.0
Other U.S. Thermal Mining
Reportable segment results
Revenues	184.6	179.8	496.0	524.1
Adjusted EBITDA	45.1	51.6	125.6	123.0
Corporate and Other (1)
Reportable segment results
Revenues	(192.9)	(15.4)	(185.6)	(7.1)
Adjusted EBITDA	$ 45.2	$ (42.5)	$ 21.2	$ (132.4)