Revenues - Revenues by Contract Duration (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,250.6	$ 1,462.7
One year or longer
Disaggregation of Revenue [Line Items]
Revenues	997.8	1,233.1
Less than one year
Disaggregation of Revenue [Line Items]
Revenues	192.2	162.3
Other
Disaggregation of Revenue [Line Items]
Revenues	60.6	67.3
Seaborne Thermal Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	251.0	201.4
Seaborne Thermal Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	171.1	177.3
Seaborne Thermal Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	79.2	23.7
Seaborne Thermal Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	0.7	0.4
Seaborne Metallurgical Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	324.5	466.2
Seaborne Metallurgical Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	232.8	397.5
Seaborne Metallurgical Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	90.9	67.8
Seaborne Metallurgical Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	0.8	0.9
Powder River Basin Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	287.3	389.3
Powder River Basin Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	280.1	343.4
Powder River Basin Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	7.2	45.8
Powder River Basin Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.1
Midwestern U.S. Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	179.1	201.7
Midwestern U.S. Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	167.8	187.6
Midwestern U.S. Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	11.2	14.0
Midwestern U.S. Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	0.1	0.1
Western U.S. Mining | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	155.7	143.7
Western U.S. Mining | Operating Segments | One year or longer
Disaggregation of Revenue [Line Items]
Revenues	146.0	127.3
Western U.S. Mining | Operating Segments | Less than one year
Disaggregation of Revenue [Line Items]
Revenues	3.7	11.0
Western U.S. Mining | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenues	6.0	5.4
Corporate and Other | Corporate, Non-Segment
Disaggregation of Revenue [Line Items]
Revenues	53.0	60.4
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	$ 53.0	$ 60.4