Self Insurance (Details) $ in Thousands	Jun. 30, 2019 USD ($) item site	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)
Self Insurance [Abstract]
Number of mining units with self-insured equipment \| site	10
Area of self-insured mining units \| item	22
Mining equipment at historical cost	$ 266,000	$ 255,000
Restricted cash	$ 4,576	$ 4,592	$ 4,234

Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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