Revenue Recognition - Revenue by Contract Duration (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 3,318.3	$ 2,881.1	$ 4,623.4
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	934.0	711.8	971.7
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	727.7	486.5	1,033.1
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	971.2	991.1	1,228.7
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	689.1	707.3	1,309.4
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	$ (3.7)	$ (15.6)	$ 80.5