Commitments and Contingencies - Schedule of Restricted Cash Deposits (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 136,032	$ 101,403
Less current portion	(17,556)	(11,977)	$ (11,427)
Long-term restricted cash	118,476	89,426	$ 85,640
Workers’ compensation and black lung obligations
Long-term Purchase Commitment [Line Items]
Total restricted cash	72,252	70,637
Reclamation-related obligations
Long-term Purchase Commitment [Line Items]
Total restricted cash	36,665	10,449
Financial payments and other performance obligations
Long-term Purchase Commitment [Line Items]
Total restricted cash	9,559	8,340
Contingent Revenue Obligation escrow
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 17,556	$ 11,977