Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 651.3	$ 846.2
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	551.2	651.4
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	86.6	192.5
Other (2)
Disaggregation of Revenue [Line Items]
Revenues	13.5	2.3
Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	419.3	487.7
Domestic | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	2.7
Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	131.9	163.7
Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	83.9	192.5
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	176.4	201.1
Seaborne Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	176.0	200.2
Seaborne Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Seaborne Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	0.4	0.9
Seaborne Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	44.1	36.5
Seaborne Thermal Mining | Domestic | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0
Seaborne Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	131.9	163.7
Seaborne Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	87.5	193.2
Seaborne Metallurgical Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Seaborne Metallurgical Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	86.6	192.5
Seaborne Metallurgical Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	0.9	0.7
Seaborne Metallurgical Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Seaborne Metallurgical Mining | Domestic | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	2.7
Seaborne Metallurgical Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Seaborne Metallurgical Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	83.9	192.5
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	228.4	266.6
Powder River Basin Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	228.4	266.6
Powder River Basin Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Powder River Basin Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Powder River Basin Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	228.4	266.6
Powder River Basin Mining | Domestic | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0
Powder River Basin Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Powder River Basin Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	149.3	192.3
Other U.S. Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	146.8	184.6
Other U.S. Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Other U.S. Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	2.5	7.7
Other U.S. Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	146.8	184.6
Other U.S. Thermal Mining | Domestic | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0
Other U.S. Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Other U.S. Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	9.7	(7.0)
Corporate and Other (1) | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other (1) | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other (1) | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	9.7	(7.0)
Corporate and Other (1) | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other (1) | Domestic | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0
Corporate and Other (1) | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other (1) | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0