Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 1,321.9	$ 723.4	$ 2,013.3	$ 1,374.7
Other Income	3.4	3.8	5.6	6.1
Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Unrealized loss recognized in income on derivatives	(31.4)	(21.1)	(318.3)	(33.6)
(Loss) gain realized in income on derivatives	(122.9)	(9.5)	(192.6)	(10.5)
Total (loss) gain recognized in income	(154.3)	(30.6)	(510.9)	(44.1)
Derivative contracts related to forecasted sales | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Unrealized loss recognized in income on derivatives	(24.5)	(23.7)	(325.5)	(25.6)
(Loss) gain realized in income on derivatives	(122.6)	(7.1)	(190.6)	(15.0)
Total (loss) gain recognized in income	(147.1)	(30.8)	(516.1)	(40.6)
Financial trading contracts | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Unrealized loss recognized in income on derivatives	(0.6)	3.8	10.2	0.8
(Loss) gain realized in income on derivatives	0.9	(2.4)	0.2	(0.1)
Total (loss) gain recognized in income	0.3	1.4	10.4	0.7
Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	808.4	603.9	1,510.7	1,155.1
Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenue		23.2		41.1
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	533.3	120.4	851.3	207.0
Other (2)
Disaggregation of Revenue [Line Items]
Revenue	(19.8)	(0.9)	(348.7)	12.6
Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	492.5	456.3	984.3	875.6
Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	315.9	147.6	526.4	279.5
Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	533.3	120.4	851.3	207.0
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenue	354.9	194.1	606.1	370.5
Seaborne Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	354.7	193.9	605.6	369.9
Seaborne Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Seaborne Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	0.2	0.2	0.5	0.6
Seaborne Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	39.8	47.5	80.2	91.6
Seaborne Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	314.9	146.4	525.4	278.3
Seaborne Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenue	533.8	121.0	855.1	208.5
Seaborne Metallurgical Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Seaborne Metallurgical Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	533.3	120.4	851.3	207.0
Seaborne Metallurgical Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	0.5	0.6	3.8	1.5
Seaborne Metallurgical Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Seaborne Metallurgical Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Seaborne Metallurgical Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	533.3	120.4	851.3	207.0
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenue	229.7	248.6	480.9	477.0
Powder River Basin Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	229.9	248.8	481.4	477.2
Powder River Basin Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Powder River Basin Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	(0.2)	(0.2)	(0.5)	(0.2)
Powder River Basin Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	229.9	248.8	481.4	477.2
Powder River Basin Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Powder River Basin Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenue	224.9	162.1	428.0	311.4
Other U.S. Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	223.8	161.2	423.7	308.0
Other U.S. Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Other U.S. Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	1.1	0.9	4.3	3.4
Other U.S. Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	222.8	160.0	422.7	306.8
Other U.S. Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	1.0	1.2	1.0	1.2
Other U.S. Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue	(21.4)	(2.4)	(356.8)	7.3
Corporate and Other (1) | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Corporate and Other (1) | Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenue	122.0	23.2	143.3	41.1
Corporate and Other (1) | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Corporate and Other (1) | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	21.4	2.4	356.8	7.3
Corporate and Other (1) | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Corporate and Other (1) | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Corporate and Other (1) | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	$ 0.0	$ 0.0	$ 0.0	$ 0.0