Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 723.4	$ 626.7	$ 1,374.7	$ 1,472.9
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	603.9	511.7	1,155.1	1,163.1
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	120.4	91.4	207.0	283.9
Other (2)
Disaggregation of Revenue [Line Items]
Revenues	(0.9)	23.6	12.6	25.9
Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	456.3	388.2	875.6	875.9
Domestic \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			2.7
Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	147.6	123.5	279.5	287.2
Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	120.4	91.4	204.3	283.9
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	194.1	162.0	370.5	363.1
Seaborne Thermal Mining \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	193.9	161.8	369.9	362.0
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	0.6	1.1
Seaborne Thermal Mining \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	47.5	38.3	91.6	74.8
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	146.4	123.5	278.3	287.2
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	121.0	91.6	208.5	284.8
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	120.4	91.4	207.0	283.9
Seaborne Metallurgical Mining \| Other (2)
Disaggregation of Revenue [Line Items]
Revenues	0.6	0.2	1.5	0.9
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	120.4	91.4	204.3	283.9
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	248.6	205.8	477.0	472.4
Powder River Basin Mining \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	248.8	205.8	477.2	472.4
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)	0.0	(0.2)	0.0
Powder River Basin Mining \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	248.8	205.8	477.2	472.4
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	162.1	152.0	311.4	344.3
Other U.S. Thermal Mining \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	161.2	144.1	308.0	328.7
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	0.9	7.9	3.4	15.6
Other U.S. Thermal Mining \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	160.0	144.1	306.8	328.7
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	1.2	0.0	1.2	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	(2.4)	15.3	7.3	8.3
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	(2.4)	15.3	7.3	8.3
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