Acquisition-Related Obligations - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations		$ 63,929	$ 79,898
Acquisition related obligations, current		33,211	33,639
Discount		3,742	4,834
Contingent Revenue Obligation
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations		37,430	52,427
Acquisition related obligations, current		14,710	14,646
Environmental Settlement Obligations
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations		11,966	13,594
Acquisition related obligations, current		5,183	6,185
Discount		2,275	2,711
Reclamation Funding Liability
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations		11,318	10,808
Acquisition related obligations, current		11,318	10,808
Discount		$ 682	$ 1,192
Subsequent Event | Contingent Revenue Obligation
Business Acquisition, Contingent Consideration [Line Items]
Payments for acquisition-related obligations	$ 15,084
Acquisition-Related Obligations, Unclaimed Distributions	$ 374