Self Insurance (Details) $ in Thousands	Dec. 31, 2018 USD ($) mi item	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Area of self-insured mining units | mi	22
Mining equipment at historical cost	$ 255,000
Restricted cash	$ 4,592	$ 3,811
High Efficiency Units [Member]
Property, Plant and Equipment [Line Items]
Number of mining units with self-insured equipment | item	10