Commitments and Contingencies - Schedule of Restricted Cash Deposits (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 131,169	$ 134,887
Less current portion	(7,104)	(12,363)	$ (21,456)
Long-term restricted cash	124,065	122,524	$ 209,041
Workers' compensation
Long-term Purchase Commitment [Line Items]
Total restricted cash	39,141	38,944
Black lung
Long-term Purchase Commitment [Line Items]
Total restricted cash	12,770	12,706
Reclamation-related obligations
Long-term Purchase Commitment [Line Items]
Total restricted cash	69,148	67,868
Financial guarantees and other
Long-term Purchase Commitment [Line Items]
Total restricted cash	3,006	3,006
Contingent revenue obligation escrow
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 7,104	$ 12,363