Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 679.0	$ 671.0	$ 2,053.7	$ 2,143.9
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	691.0	599.9	1,846.1	1,763.0
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	176.8	78.4	383.8	362.3
Coal [Member]
Disaggregation of Revenue [Line Items]
Revenues	923.0	665.0	2,327.0	2,093.0
Other (2)
Disaggregation of Revenue [Line Items]
Revenues	(188.8)	(7.3)	(176.2)	18.6
Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	469.4	474.2	1,345.0	1,350.1
Domestic | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			2.7
Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	221.6	125.7	501.1	412.9
Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	176.8	78.4	381.1	362.3
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	260.7	163.0	631.2	526.1
Seaborne Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	260.5	162.8	630.4	524.8
Seaborne Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	0.2	0.2	0.8	1.3
Seaborne Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	41.1	37.1	132.7	111.9
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	219.4	125.7	497.7	412.9
Seaborne Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	179.5	78.8	388.0	363.6
Seaborne Metallurgical Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Metallurgical Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	176.8	78.4	383.8	362.3
Seaborne Metallurgical Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	2.7	0.4	4.2	1.3
Seaborne Metallurgical Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	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	0.0	0.0
Seaborne Metallurgical Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	176.8	78.4	381.1	362.3
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	247.1	264.8	724.1	737.2
Powder River Basin Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	247.3	264.3	724.5	736.7
Powder River Basin Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Powder River Basin Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	(0.2)	0.5	(0.4)	0.5
Powder River Basin Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	247.3	264.3	724.5	736.7
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	0.0	0.0
Powder River Basin Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	184.6	179.8	496.0	524.1
Other U.S. Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	183.2	172.8	491.2	501.5
Other U.S. Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Other U.S. Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	1.4	7.0	4.8	22.6
Other U.S. Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	181.0	172.8	487.8	501.5
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	2.2	0.0	3.4	0.0
Other U.S. Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	(192.9)	(15.4)	(185.6)	(7.1)
Corporate and Other (1) | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1) | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1) | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	192.9	15.4	(185.6)	7.1
Corporate and Other (1) | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	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	0.0	0.0
Corporate and Other (1) | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0