Acquisition-Related Obligations (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Option, Quantitative Disclosures [Line Items]
Discount	$ (3,742)	$ (4,834)
Total acquisition-related obligations	63,929	79,898
Less current portion	(33,211)	(33,639)
Acquisition-related obligations, net of current portion	30,718	46,259
Contingent Revenue Obligation
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	37,430	52,427
Total acquisition-related obligations	37,430	52,427
Less current portion	(14,710)	(14,646)
Environmental Settlement Obligations
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	14,241	16,305
Discount	(2,275)	(2,711)
Total acquisition-related obligations	11,966	13,594
Less current portion	(5,183)	(6,185)
Reclamation Funding Liability
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	12,000	12,000
Discount	(682)	(1,192)
Total acquisition-related obligations	11,318	10,808
Less current portion	(11,318)	(10,808)
UMWA Funds Settlement Liability
Fair Value, Option, Quantitative Disclosures [Line Items]
Total acquisition-related obligations - long-term, gross	$ 4,000	$ 4,000