Revenue Recognition - Revenues by Contract Duration (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 626.7	$ 1,149.0	$ 1,472.9	$ 2,399.6
One year or longer
Disaggregation of Revenue [Line Items]
Revenues	481.4	953.8	1,148.8	1,951.6
Less than one year
Disaggregation of Revenue [Line Items]
Revenues	121.7	141.4	298.2	333.6
Other
Disaggregation of Revenue [Line Items]
Revenues	23.6	53.8	25.9	114.4
Seaborne Thermal Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	162.0	220.2	363.1	471.2
Seaborne Thermal Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	79.5	150.6	178.8	321.7
Seaborne Thermal Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	82.3	69.3	183.2	148.5
Seaborne Thermal Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.2	0.3	1.1	1.0
Seaborne Metallurgical Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	91.6	290.9	284.8	615.4
Seaborne Metallurgical Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	64.1	242.7	204.2	475.5
Seaborne Metallurgical Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	27.3	47.6	79.7	138.5
Seaborne Metallurgical Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.2	0.6	0.9	1.4
Powder River Basin Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	205.8	282.6	472.4	569.9
Powder River Basin Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	195.7	269.7	439.1	549.8
Powder River Basin Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	10.1	12.8	33.3	20.0
Powder River Basin Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.1	0.0	0.1
Other U.S. Thermal Mining [Member] \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	152.0	309.6	344.3	644.4
Other U.S. Thermal Mining [Member] \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	142.1	290.8	326.7	604.6
Other U.S. Thermal Mining [Member] \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	2.0	11.7	2.0	26.6
Other U.S. Thermal Mining [Member] \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	7.9	7.1	15.6	13.2
Corporate and Other \| Corporate, Non-Segment
Disaggregation of Revenue [Line Items]
Revenues	15.3	45.7	8.3	98.7
Corporate and Other \| Corporate, Non-Segment \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other \| Corporate, Non-Segment \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other \| Corporate, Non-Segment \| Other
Disaggregation of Revenue [Line Items]
Revenues	$ 15.3	$ 45.7	$ 8.3	$ 98.7