Revenue Recognition - Revenue by Product Type and Market (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
Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Unrealized (loss) gain recognized in income on derivatives	(121.1)	(27.4)	51.8
Coal Contract and Physical commodity purchase / sale contracts | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Unrealized (loss) gain recognized in income on derivatives	(113.7)	(34.5)	50.6
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
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	2,583.1	2,379.4	3,464.7
Thermal coal | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	932.5	710.3	970.3
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	970.7	993.9	1,208.9
Thermal coal | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	679.9	675.2	1,285.5
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	1,814.1	1,814.6	2,631.0
Thermal coal | Domestic | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	173.5	145.5	147.9
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	970.7	993.9	1,208.9
Thermal coal | Domestic | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	669.9	675.2	1,274.2
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	769.0	564.8	833.7
Thermal coal | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	759.0	564.8	822.4
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	10.0	0.0	11.3
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	719.8	484.3	1,030.0
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	719.8	484.3	1,030.0
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	719.8	484.3	1,030.0
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	719.8	484.3	1,030.0
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 (2)
Disaggregation of Revenue [Line Items]
Revenues	15.4	17.4	128.7
Other (2) | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	1.5	1.5	1.4
Other (2) | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	7.9	2.2	3.1
Other (2) | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	0.5	(2.8)	19.8
Other (2) | Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	9.2	32.1	23.9
Other (2) | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	(3.7)	(15.6)	80.5
Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenues		(28.5)	(18.8)
Trading and Brokerage Coal Deliveries | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	139.5	(28.9)	(17.6)
Coal
Disaggregation of Revenue [Line Items]
Revenues	$ 3,442.0	$ 2,835.0	$ 4,477.0