Revenue Recognition - Revenues by Contract Duration (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 671.0	$ 1,106.4	$ 2,143.9	$ 3,506.0
One year or longer
Disaggregation of Revenue [Line Items]
Revenues	587.5	954.8	1,736.3	2,906.4
Less than one year
Disaggregation of Revenue [Line Items]
Revenues	90.8	163.0	389.0	496.6
Other
Disaggregation of Revenue [Line Items]
Revenues	(7.3)	(11.4)	18.6	103.0
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	163.0	249.5	526.1	720.7
Seaborne Thermal Mining | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	93.6	158.5	272.4	480.2
Seaborne Thermal Mining | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	69.2	90.9	252.4	239.4
Seaborne Thermal Mining | Other
Disaggregation of Revenue [Line Items]
Revenues	0.2	0.1	1.3	1.1
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues	78.8	216.3	363.6	831.7
Seaborne Metallurgical Mining | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	65.2	193.3	269.4	668.8
Seaborne Metallurgical Mining | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	13.2	22.1	92.9	160.6
Seaborne Metallurgical Mining | Other
Disaggregation of Revenue [Line Items]
Revenues	0.4	0.9	1.3	2.3
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues	264.8	333.6	737.2	903.5
Powder River Basin Mining | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	257.4	287.8	696.5	837.6
Powder River Basin Mining | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	6.9	45.8	40.2	65.8
Powder River Basin Mining | Other
Disaggregation of Revenue [Line Items]
Revenues	0.5	0.0	0.5	0.1
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues	179.8	326.4	524.1	970.8
Other U.S. Thermal Mining | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	171.3	315.2	498.0	919.8
Other U.S. Thermal Mining | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	1.5	4.2	3.5	30.8
Other U.S. Thermal Mining | Other
Disaggregation of Revenue [Line Items]
Revenues	7.0	7.0	22.6	20.2
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	(15.4)	(19.4)	(7.1)	79.3
Corporate and Other | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other | Other
Disaggregation of Revenue [Line Items]
Revenues	$ (15.4)	$ (19.4)	$ (7.1)	$ 79.3