Segment Information Segment Results (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reportable segment results
Revenues	$ 1,149.0	$ 1,309.4	$ 2,399.6	$ 2,772.1
Total Adjusted EBITDA	228.0	369.6	481.9	733.5
Net gain on disposals	0.2	(1.6)	1.7	29.0
Iffley Land Transaction
Reportable segment results
Net gain on disposals				20.6
Red Mountain Joint Venture
Reportable segment results
Net gain on disposals				7.1
Seaborne Thermal Mining | Operating Segments
Reportable segment results
Revenues	220.2	267.4	471.2	468.8
Total Adjusted EBITDA	74.4	107.6	169.1	169.2
Seaborne Metallurgical Mining | Operating Segments
Reportable segment results
Revenues	290.9	417.5	615.4	883.7
Total Adjusted EBITDA	57.4	158.5	143.2	324.9
Powder River Basin Mining | Operating Segments
Reportable segment results
Revenues	282.6	321.5	569.9	710.8
Total Adjusted EBITDA	40.2	62.0	76.6	136.5
Midwestern U.S. Mining | Operating Segments
Reportable segment results
Revenues	167.5	197.5	346.6	399.2
Total Adjusted EBITDA	30.7	42.0	64.0	73.2
Western U.S. Mining | Operating Segments
Reportable segment results
Revenues	142.1	139.6	297.8	283.3
Total Adjusted EBITDA	52.4	33.9	95.0	65.9
Corporate and Other | Corporate, Non-Segment
Reportable segment results
Revenues	45.7	(34.1)	98.7	26.3
Total Adjusted EBITDA	$ (27.1)	$ (34.4)	$ (66.0)	$ (36.2)