Revenues - Revenue by Contract Duration (Details) - USD ($) $ in Millions	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,117.4	$ 1,106.4	$ 1,149.0	$ 1,250.6	$ 1,397.1	$ 1,412.6	$ 1,309.4	$ 1,462.7	$ 1,326.2	$ 4,252.6	$ 4,623.4	$ 5,581.8
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									224.8	772.5	971.7	1,099.2
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									328.9	1,221.0	1,033.1	1,553.0
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3	1,178.7	1,228.7	1,424.8
Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2	592.3	669.7	801.0
Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									149.7	440.7	639.7	592.0
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									35.3	47.4	80.5	111.8
One year or longer
Disaggregation of Revenue [Line Items]
Revenues									1,054.9	3,358.3	3,759.6	4,427.3
Less than one year
Disaggregation of Revenue [Line Items]
Revenues									215.2	830.0	735.1	1,010.0
Other
Disaggregation of Revenue [Line Items]
Revenues									56.1	64.3	128.7	144.5
Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									224.8	772.5	971.7	1,099.2
Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									328.9	1,221.0	1,033.1	1,553.0
Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3	1,178.7	1,228.7	1,424.8
Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2	592.3	669.7	801.0
Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									149.7	440.7	639.7	592.0
Operating Segments | One year or longer | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									134.1	503.0	589.2	799.5
Operating Segments | One year or longer | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									240.6	867.1	828.6	1,036.7
Operating Segments | One year or longer | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									357.7	1,023.1	1,087.6	1,283.9
Operating Segments | One year or longer | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2	560.5	644.3	775.4
Operating Segments | One year or longer | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									129.3	404.6	609.9	531.8
Operating Segments | Less than one year | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									90.4	269.0	381.1	298.5
Operating Segments | Less than one year | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									84.0	353.9	201.4	511.9
Operating Segments | Less than one year | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									36.6	151.2	121.3	140.9
Operating Segments | Less than one year | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	31.5	24.9	25.3
Operating Segments | Less than one year | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									4.2	24.4	6.4	33.4
Operating Segments | Other | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.3	0.5	1.4	1.2
Operating Segments | Other | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									4.3	0.0	3.1	4.4
Operating Segments | Other | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	4.4	19.8	0.0
Operating Segments | Other | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.3	0.5	0.3
Operating Segments | Other | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									16.2	11.7	23.4	26.8
Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									35.3	47.4	80.5	111.8
Corporate, Non-Segment | One year or longer | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Corporate, Non-Segment | Less than one year | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Corporate, Non-Segment | Other | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									$ 35.3	$ 47.4	$ 80.5	$ 111.8