Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 5,520,656	$ 5,408,577
Less: Accumulated Depreciation, Depletion and Amortization	3,603,830	3,448,495
Total Property, Plant and Equipment - Net	1,916,826	1,960,082
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	3,429,950	3,330,755
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	904,992	898,628
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	487,506	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	$ 331,948	$ 331,863