Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,881.1	$ 4,623.4	$ 5,581.8
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	711.8	971.7	1,099.2
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	486.5	1,033.1	1,553.0
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	991.1	1,228.7	1,424.8
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	707.3	1,309.4	1,393.0
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	(15.6)	80.5	111.8
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	2,379.4	3,464.7	3,888.7
Thermal coal | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	710.3	970.3	1,098.0
Thermal coal | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	993.9	1,208.9	1,424.8
Thermal coal | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	675.2	1,285.5	1,365.9
Thermal coal | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal | Domestic
Disaggregation of Revenue [Line Items]
Revenues	1,814.6	2,631.0	2,920.3
Thermal coal | Domestic | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	145.5	147.9	153.0
Thermal coal | Domestic | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal | Domestic | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	993.9	1,208.9	1,424.8
Thermal coal | Domestic | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	675.2	1,274.2	1,342.5
Thermal coal | Domestic | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal | Export
Disaggregation of Revenue [Line Items]
Revenues	564.8	833.7	968.4
Thermal coal | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	564.8	822.4	945.0
Thermal coal | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal | Export | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	11.3	23.4
Thermal coal | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	484.3	1,030.0	1,548.6
Metallurgical coal | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	484.3	1,030.0	1,548.6
Metallurgical coal | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal | Export
Disaggregation of Revenue [Line Items]
Revenues	484.3	1,030.0	1,548.6
Metallurgical coal | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	484.3	1,030.0	1,548.6
Metallurgical coal | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal | Export | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenues	17.4	128.7	144.5
Other | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	1.5	1.4	1.2
Other | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	2.2	3.1	4.4
Other | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	(2.8)	19.8	0.0
Other | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	32.1	23.9	27.1
Other | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	$ (15.6)	$ 80.5	$ 111.8