Segment and Geographic Information - Reconciliation of Assets by Segment to Consolidated Total Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 4,667.1	$ 6,542.8	$ 7,423.7
Property, plant, equipment and mine development, net	3,051.1	4,679.1	5,207.0
Operating lease right-of-use assets	49.9	82.4
Seaborne Mining
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,763.0	2,001.3	2,044.6
Property, plant, equipment and mine development, net	1,347.3	1,610.9	1,661.3
Operating lease right-of-use assets	30.8	32.1
U.S. Thermal Mining
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,345.3	3,044.8	3,481.7
Property, plant, equipment and mine development, net	1,258.8	2,776.9	3,180.4
Operating lease right-of-use assets	3.5	30.3
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,558.8	1,496.7	1,897.4
Property, plant, equipment and mine development, net	445.0	291.3	$ 365.3
Operating lease right-of-use assets	$ 15.6	$ 20.0