Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 5,484,648	$ 5,408,577
Less: Accumulated Depreciation, Depletion and Amortization	3,553,045	3,448,495
Total Property, Plant and Equipment - Net	1,931,603	1,960,082
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	3,397,535	3,330,755
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	904,573	898,628
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	484,502	481,090
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	366,265	366,241
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 331,773	$ 331,863