Acquisition-Related Obligations (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Option, Quantitative Disclosures [Line Items]
Discount	$ (1,952)	$ (4,834)
Total acquisition-related obligations	37,492	79,898
Less current portion	(18,206)	(33,639)
Acquisition-related obligations, net of current portion	19,286	46,259
Contingent Revenue Obligation
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	24,292	52,427
Total acquisition-related obligations	24,292	52,427
Less current portion	(10,511)	(14,646)
Environmental Settlement Obligations
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	11,152	16,305
Discount	(1,478)	(2,711)
Total acquisition-related obligations	9,674	13,594
Less current portion	(5,694)	(6,185)
Reclamation Funding Liability
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	0	12,000
Discount		(1,192)
Less current portion		(10,808)
UMWA Funds Settlement Liability
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	$ 4,000	$ 4,000