Acquisition-Related Obligations (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Option, Quantitative Disclosures [Line Items]
Discount	$ (2,607)	$ (4,834)
Total acquisition-related obligations	48,302	79,898
Less current portion	(30,019)	(33,639)
Acquisition-related obligations, net of current portion	18,283	46,259
Contingent Revenue Obligation
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	20,668	52,427
Total acquisition-related obligations	20,668	52,427
Less current portion	(9,922)	(14,646)
Environmental Settlement Obligations
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	14,241	16,305
Discount	(1,825)	(2,711)
Total acquisition-related obligations	12,416	13,594
Less current portion	(6,245)	(6,185)
Reclamation Funding Liability
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	12,000	12,000
Discount	(148)	(1,192)
Total acquisition-related obligations	11,852	10,808
Less current portion	(11,852)	(10,808)
UMWA Funds Settlement Liability
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	$ 4,000	$ 4,000