Segment and Geographic Information Reconciliation of Assets by Segment to Consolidated Total Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 6,542.8	$ 7,423.7	$ 8,181.2
Property, plant, equipment and mine development, net	4,679.1	5,207.0	5,111.9
Operating lease right-of-use assets	82.4	0.0
Seaborne Mining
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	2,001.3	2,044.6	2,339.6
Property, plant, equipment and mine development, net	1,610.9	1,661.3	1,501.7
Operating lease right-of-use assets	32.1
U.S. Thermal Mining
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	3,044.8	3,481.7	3,846.5
Property, plant, equipment and mine development, net	2,776.9	3,180.4	3,361.0
Operating lease right-of-use assets	30.3
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,496.7	1,897.4	1,995.1
Property, plant, equipment and mine development, net	291.3	$ 365.3	$ 249.2
Operating lease right-of-use assets	$ 20.0