Fixed Assets and Mineral Leaseholds (Mineral Leaseholds) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fixed Assets And Asset Retirement Obligations [Abstract]
Mineral leaseholds	$ 566,019	$ 566,019
Less: Accumulated depletion	(4,426)	(1,513)
Mineral leaseholds, net	$ 561,593	$ 564,506

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
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61797-109447

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 360
-Section 25
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=96866604&loc=d3e64895-109465

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