Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 5,655,655	$ 5,552,404
Less: Accumulated Depreciation, Depletion and Amortization	3,744,633	3,649,281
Total Property, Plant and Equipment - Net	1,911,022	1,903,123
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	3,546,964	3,458,655
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	909,303	906,343
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	503,843	492,806
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	366,260	366,260
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 329,285	$ 328,340