Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 4,981.9	$ 3,318.3	$ 2,881.1
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	1,345.6	934.0	711.8
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	1,616.9	727.7	486.5
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	1,065.5	971.2	991.1
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	952.2	689.1	707.3
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	1.7	(3.7)	(15.6)
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	3,358.3	2,583.1	2,379.4
Thermal coal \| Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	1,344.9	932.5	710.3
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	1,066.0	970.7	993.9
Thermal coal \| Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	947.4	679.9	675.2
Thermal coal \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal \| Domestic
Disaggregation of Revenue [Line Items]
Revenues	2,177.5	1,814.1	1,814.6
Thermal coal \| Domestic \| Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	167.6	173.5	145.5
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	1,066.0	970.7	993.9
Thermal coal \| Domestic \| Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	943.9	669.9	675.2
Thermal coal \| Domestic \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Thermal coal \| Export
Disaggregation of Revenue [Line Items]
Revenues	1,180.8	769.0	564.8
Thermal coal \| Export \| Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	1,177.3	759.0	564.8
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	3.5	10.0	0.0
Thermal coal \| Export \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	1,610.8	719.8	484.3
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	1,610.8	719.8	484.3
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 (1)
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Metallurgical coal \| Export
Disaggregation of Revenue [Line Items]
Revenues	1,610.8	719.8	484.3
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	1,610.8	719.8	484.3
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 (1)
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0
Other (2)
Disaggregation of Revenue [Line Items]
Revenues	12.8	15.4	17.4
Other (2) \| Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	0.7	1.5	1.5
Other (2) \| Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	6.1	7.9	2.2
Other (2) \| Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	(0.5)	0.5	(2.8)
Other (2) \| Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	4.8	9.2	32.1
Other (2) \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	1.7	(3.7)	(15.6)
Trading and Brokerage Coal Deliveries \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	$ 470.7	$ 140.3	$ (28.6)