Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 8,852,822	$ 8,730,802
Less: Accumulated Depreciation, Depletion and Amortization	4,477,329	4,343,920
Total Property, Plant and Equipment—Net	4,375,493	4,386,882
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	4,979,427	4,876,926
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	2,244,359	2,241,915
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	608,183	591,146
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	690,337	690,845
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 330,516	$ 329,970