Segment and Geographic Information - Reconciliation of Assets by Segment to Consolidated Total Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 5,962.1	$ 5,610.8	$ 4,949.8
Property, plant, equipment and mine development, net	2,844.1	2,865.0	2,950.6
Operating lease right-of-use assets	61.9	26.9	35.5
Seaborne
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	2,088.2	1,632.6	1,669.6
Property, plant, equipment and mine development, net	1,565.1	1,220.7	1,298.8
Operating lease right-of-use assets	33.1	20.5	19.2
U.S. Thermal
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,373.2	1,359.4	1,318.5
Property, plant, equipment and mine development, net	1,149.2	1,217.5	1,209.5
Operating lease right-of-use assets	25.0	0.5	3.3
Corporate and Other (1)
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	2,500.7	2,618.8	1,961.7
Property, plant, equipment and mine development, net	129.8	426.8	442.3
Operating lease right-of-use assets	$ 3.8	$ 5.9	$ 13.0