Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 5,442,735	$ 5,408,577
Less: Accumulated Depreciation, Depletion and Amortization	3,502,386	3,448,495
Total Property, Plant and Equipment - Net	1,940,349	1,960,082
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	3,359,934	3,330,755
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	901,364	898,628
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	482,641	481,090
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	366,241	366,241
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 332,555	$ 331,863