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Segment and Geographic Information - Reconciliation of Assets by Segment to Consolidated Total Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Segment Reporting, Asset Reconciling Item [Line Items]      
Total assets $ 4,949.8 $ 4,667.1 $ 6,542.8
Property, plant, equipment and mine development, net 2,950.6 3,051.1 4,679.1
Operating lease right-of-use assets 35.5 49.9 82.4
Seaborne Mining      
Segment Reporting, Asset Reconciling Item [Line Items]      
Total assets 1,669.6 1,763.0 2,001.3
Property, plant, equipment and mine development, net 1,298.8 1,347.3 1,610.9
Operating lease right-of-use assets 19.2 30.8 32.1
U.S. Thermal Mining      
Segment Reporting, Asset Reconciling Item [Line Items]      
Total assets 1,318.5 1,345.3 3,044.8
Property, plant, equipment and mine development, net 1,209.5 1,258.8 2,776.9
Operating lease right-of-use assets 3.3 3.5 30.3
Corporate and Other      
Segment Reporting, Asset Reconciling Item [Line Items]      
Total assets 1,961.7 1,558.8 1,496.7
Property, plant, equipment and mine development, net 442.3 445.0 291.3
Operating lease right-of-use assets $ 13.0 $ 15.6 $ 20.0