Acquisition-Related Obligations - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations	$ 48,302	$ 79,898
Acquisition related obligations, current	30,019	33,639
Discount	2,607	4,834
Contingent Revenue Obligation
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations	20,668	52,427
Acquisition related obligations, current	9,922	14,646
Payments for acquisition-related obligations	15,084
Payments for acquisition-related obligations, unclaimed	374
Environmental Settlement Obligations
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations	12,416	13,594
Acquisition related obligations, current	6,245	6,185
Discount	1,825	2,711
Reclamation Funding Liability
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations	11,852	10,808
Acquisition related obligations, current	11,852	10,808
Discount	$ 148	$ 1,192