Fixed Assets and Asset Retirement Obligations (Schedule of Mineral Leaseholds) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Mineral leaseholds	$ 566,019	$ 566,019
Less: Accumulated depletion	(13,444)	(10,194)
Mineral leaseholds, net	$ 552,575	$ 555,825

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

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Name:	us-gaap_MineralPropertiesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant