Commitments and Contingencies - Schedule of Restricted Cash Deposits (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 116,929	$ 134,887
Less current portion	(15,114)	(12,363)	$ (19,307)
Long-term restricted cash	101,815	122,524	$ 233,488
Workers’ compensation
Long-term Purchase Commitment [Line Items]
Total restricted cash	39,097	38,944
Black lung
Long-term Purchase Commitment [Line Items]
Total restricted cash	12,756	12,706
Reclamation-related obligations
Long-term Purchase Commitment [Line Items]
Total restricted cash	46,953	67,868
Financial guarantees and other
Long-term Purchase Commitment [Line Items]
Total restricted cash	3,009	3,006
Contingent revenue obligation escrow
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 15,114	$ 12,363