Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 5,552,404	$ 5,408,577
Less—Accumulated Depreciation, Depletion and Amortization	3,649,281	3,448,495
Total Property, Plant and Equipment—Net	1,903,123	1,960,082
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	3,458,655	3,330,755
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	906,343	898,628
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	492,806	481,090
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	366,260	366,241
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 328,340	$ 331,863