Revenue Recognition - Revenues by Contract Duration (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,149.0	$ 1,309.4	$ 2,399.6	$ 2,772.1
One year or longer
Disaggregation of Revenue [Line Items]
Revenues	953.8	1,059.7	1,951.6	2,292.8
Less than one year
Disaggregation of Revenue [Line Items]
Revenues	141.4	279.1	333.6	441.4
Other
Disaggregation of Revenue [Line Items]
Revenues	53.8	(29.4)	114.4	37.9
Seaborne Thermal Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	220.2	267.4	471.2	468.8
Seaborne Thermal Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	150.6	173.2	321.7	350.5
Seaborne Thermal Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	69.3	94.0	148.5	117.7
Seaborne Thermal Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.3	0.2	1.0	0.6
Seaborne Metallurgical Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	290.9	417.5	615.4	883.7
Seaborne Metallurgical Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	242.7	255.1	475.5	652.6
Seaborne Metallurgical Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	47.6	162.1	138.5	229.9
Seaborne Metallurgical Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.6	0.3	1.4	1.2
Powder River Basin Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	282.6	321.5	569.9	710.8
Powder River Basin Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	269.7	310.8	549.8	654.2
Powder River Basin Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	12.8	10.7	20.0	56.5
Powder River Basin Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.1	0.0	0.1	0.1
Midwestern U.S. Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	167.5	197.5	346.6	399.2
Midwestern U.S. Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	158.2	194.1	326.0	381.7
Midwestern U.S. Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	9.3	3.4	20.5	17.4
Midwestern U.S. Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.1	0.1
Western U.S. Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	142.1	139.6	297.8	283.3
Western U.S. Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	132.6	126.5	278.6	253.8
Western U.S. Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	2.4	8.9	6.1	19.9
Western U.S. Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	7.1	4.2	13.1	9.6
Corporate and Other \| Corporate, Non-Segment
Disaggregation of Revenue [Line Items]
Revenues	45.7	(34.1)	98.7	26.3
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	$ 45.7	$ (34.1)	$ 98.7	$ 26.3