Commitments and Contingencies (Details) $ in Thousands, T in Millions	6 Months Ended
	Jun. 30, 2021 USD ($) operating_lease T	Mar. 31, 2021 USD ($)
Unrecorded Unconditional Purchase Obligation [Line Items]
Royalty payment, percentage of net profit	4.00%
Royalty payment, annual advance	$ 120
Royalty payment, additional incremental payment	$ 120
Royalty payment, annual tons mined threshold | T	5.0
Royalty payment, maximum lease payments	$ 7,600
Payments to acquire royalty interests in mining properties	2,800
Other assets, noncurrent
Unrecorded Unconditional Purchase Obligation [Line Items]
Payments to acquire royalty interests in mining properties	500
Inventories | Prepaids and other
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded unconditional purchase obligation, purchases	1,900
Inventories | Inventories
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded unconditional purchase obligation, purchases	$ 500
Mobile mining equipment
Unrecorded Unconditional Purchase Obligation [Line Items]
Operating lease, number of leases | operating_lease	2
Operating lease, remaining lease payments	$ 900
Large wheel loader lease
Unrecorded Unconditional Purchase Obligation [Line Items]
Operating lease, remaining lease payments	8,500
Operating lease, monthly payment		$ 100
Operating lease, term		4 years
Large wheel loader lease | Maintenance
Unrecorded Unconditional Purchase Obligation [Line Items]
Operating lease, remaining lease payments	3,700
Office space and corporate housing
Unrecorded Unconditional Purchase Obligation [Line Items]
Operating lease, annual rent expense	$ 100