Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 5,764,081	$ 5,552,404
Less—Accumulated Depreciation, Depletion and Amortization	3,842,382	3,649,281
Total Property, Plant and Equipment—Net	1,921,699	1,903,123
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	3,633,741	3,458,655
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	913,819	906,343
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	521,334	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	$ 328,927	$ 328,340