Commitments and Contingencies - Schedule of Restricted Cash Deposits (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 113,650	$ 134,887
Less current portion	(3,720)	(12,363)	$ (34,309)
Long-term restricted cash	109,930	122,524	$ 216,568
Workers' compensation
Long-term Purchase Commitment [Line Items]
Total restricted cash	39,135	38,944
Black lung
Long-term Purchase Commitment [Line Items]
Total restricted cash	12,768	12,706
Reclamation-related obligations
Long-term Purchase Commitment [Line Items]
Total restricted cash	55,019	67,868
Financial guarantees and other
Long-term Purchase Commitment [Line Items]
Total restricted cash	3,008	3,006
Contingent revenue obligation escrow
Long-term Purchase Commitment [Line Items]
Total restricted cash	$ 3,720	$ 12,363