Acquisition-Related Obligations - Narrative (Details) - USD ($)	3 Months Ended
	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations		$ 37,492,000	$ 79,898,000
Acquisition related obligations, current		18,206,000	33,639,000
Discount		1,952,000	4,834,000
Contingent Revenue Obligation
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations		24,292,000	52,427,000
Acquisition related obligations, current		10,511,000	14,646,000
Payments for acquisition-related obligations	$ 15,084,000
Payments for acquisition-related obligations, unclaimed	$ 374,000
Environmental Settlement Obligations
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations		9,674,000	13,594,000
Acquisition related obligations, current		5,694,000	6,185,000
Discount		1,478,000	2,711,000
Reclamation Funding Liability
Business Acquisition, Contingent Consideration [Line Items]
Acquisition related obligations, current			10,808,000
Remaining payments for acquisition-related obligations		$ 0
Discount			$ 1,192,000