Revenue Recognition - Revenues by Contract Duration (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 846.2	$ 1,250.6
One year or longer
Disaggregation of Revenue [Line Items]
Revenues	667.4	997.8
Less than one year
Disaggregation of Revenue [Line Items]
Revenues	176.5	192.2
Other
Disaggregation of Revenue [Line Items]
Revenues	2.3	60.6
Seaborne Thermal Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	201.1	251.0
Seaborne Thermal Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	99.3	171.1
Seaborne Thermal Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	100.9	79.2
Seaborne Thermal Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	0.9	0.7
Seaborne Metallurgical Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	193.2	324.5
Seaborne Metallurgical Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	140.1	232.8
Seaborne Metallurgical Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	52.4	90.9
Seaborne Metallurgical Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	0.7	0.8
Powder River Basin Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	266.6	287.3
Powder River Basin Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	243.4	280.1
Powder River Basin Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	23.2	7.2
Powder River Basin Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Other U.S. Thermal Mining [Member] | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	192.3	334.8
Other U.S. Thermal Mining [Member] | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	184.6	313.8
Other U.S. Thermal Mining [Member] | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	0.0	14.9
Other U.S. Thermal Mining [Member] | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	7.7	6.1
Corporate and Other | Corporate, Non-Segment
Disaggregation of Revenue [Line Items]
Revenues	(7.0)	53.0
Corporate and Other | Corporate, Non-Segment | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other | Corporate, Non-Segment | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other | Corporate, Non-Segment | Other
Disaggregation of Revenue [Line Items]
Revenues	$ (7.0)	$ 53.0