Revenues - Revenue by Product Type and Market (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
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
Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									35.3	47.4	80.5	111.8
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues									945.5	2,967.3	3,464.7	3,888.7
Thermal coal | Domestic
Disaggregation of Revenue [Line Items]
Revenues									748.3	2,258.7	2,631.0	2,920.3
Thermal coal | Export
Disaggregation of Revenue [Line Items]
Revenues									197.2	708.6	833.7	968.4
Thermal coal | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									224.5	772.0	970.3	1,098.0
Thermal coal | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Thermal coal | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3	1,174.3	1,208.9	1,424.8
Thermal coal | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2	592.0	669.2	800.7
Thermal coal | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									133.5	429.0	616.3	565.2
Thermal coal | Operating Segments | Domestic | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									27.3	87.9	147.9	153.0
Thermal coal | Operating Segments | Domestic | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Thermal coal | Operating Segments | Domestic | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3	1,174.3	1,208.9	1,424.8
Thermal coal | Operating Segments | Domestic | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2	586.7	669.2	799.4
Thermal coal | Operating Segments | Domestic | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									133.5	409.8	605.0	543.1
Thermal coal | Operating Segments | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									197.2	684.1	822.4	945.0
Thermal coal | Operating Segments | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Thermal coal | Operating Segments | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Thermal coal | Operating Segments | Export | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	5.3	0.0	1.3
Thermal coal | Operating Segments | Export | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	19.2	11.3	22.1
Thermal coal | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Thermal coal | Corporate, Non-Segment | Domestic | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Thermal coal | Corporate, Non-Segment | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues									324.6	1,221.0	1,030.0	1,548.6
Metallurgical coal | Export
Disaggregation of Revenue [Line Items]
Revenues									324.6	1,221.0	1,030.0	1,548.6
Metallurgical coal | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									324.6	1,221.0	1,030.0	1,548.6
Metallurgical coal | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Operating Segments | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Operating Segments | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									324.6	1,221.0	1,030.0	1,548.6
Metallurgical coal | Operating Segments | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Operating Segments | Export | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Operating Segments | Export | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Metallurgical coal | Corporate, Non-Segment | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenues									56.1	64.3	128.7	144.5
Other | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.3	0.5	1.4	1.2
Other | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									4.3	0.0	3.1	4.4
Other | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	4.4	19.8	0.0
Other | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.3	0.5	0.3
Other | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									16.2	11.7	23.4	26.8
Other | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									$ 35.3	$ 47.4	$ 80.5	$ 111.8