Revenue Recognition - Revenues by Contract Duration (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,106.4	$ 1,412.6	$ 3,506.0	$ 4,184.7
One year or longer
Disaggregation of Revenue [Line Items]
Revenues	954.8	1,116.9	2,906.4	3,409.7
Less than one year
Disaggregation of Revenue [Line Items]
Revenues	163.0	292.3	496.6	733.7
Other
Disaggregation of Revenue [Line Items]
Revenues	(11.4)	3.4	103.0	41.3
Seaborne Thermal Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	249.5	305.1	720.7	773.9
Seaborne Thermal Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	158.5	235.3	480.2	585.8
Seaborne Thermal Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	90.9	69.8	239.4	187.5
Seaborne Thermal Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.1	0.0	1.1	0.6
Seaborne Metallurgical Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	216.3	370.3	831.7	1,254.0
Seaborne Metallurgical Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	193.3	199.5	668.8	852.1
Seaborne Metallurgical Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	22.1	169.9	160.6	399.8
Seaborne Metallurgical Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.9	0.9	2.3	2.1
Powder River Basin Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	333.6	373.7	903.5	1,084.5
Powder River Basin Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	287.8	330.2	837.6	984.4
Powder River Basin Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	45.8	43.5	65.8	100.0
Powder River Basin Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.1	0.1
Midwestern U.S. Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	176.0	208.5	522.6	607.7
Midwestern U.S. Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	171.6	205.3	497.6	587.0
Midwestern U.S. Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	4.2	3.1	24.7	20.5
Midwestern U.S. Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.2	0.1	0.3	0.2
Western U.S. Mining \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	150.4	156.1	448.2	439.4
Western U.S. Mining \| Operating Segments \| One year or longer
Disaggregation of Revenue [Line Items]
Revenues	143.6	146.6	422.2	400.4
Western U.S. Mining \| Operating Segments \| Less than one year
Disaggregation of Revenue [Line Items]
Revenues	0.0	6.0	6.1	25.9
Western U.S. Mining \| Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Revenues	6.8	3.5	19.9	13.1
Corporate and Other \| Corporate, Non-Segment
Disaggregation of Revenue [Line Items]
Revenues	(19.4)	(1.1)	79.3	25.2
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	$ (19.4)	$ (1.1)	$ 79.3	$ 25.2