Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Mineral Leaseholds) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fixed Assets And Asset Retirement Obligations [Abstract]
Mineral leaseholds	$ 566,019	$ 566,019
Less: Accumulated depletion	(14,356)	(13,444)
Mineral leaseholds, net of accumulated depletion	$ 551,663	$ 552,575

Carrying amount at the balance sheet date of mineral properties, gross of adjustments.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Name:	us-gaap_MineralPropertiesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant