Self Insurance (Details) $ in Thousands	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Area of self-insured mining units | item	22
Mining equipment at historical cost | $	$ 273,000	$ 255,000
Restricted cash | $	$ 4,512	$ 4,592
High Efficiency Units [Member]
Property, Plant and Equipment [Line Items]
Number of mining units with self-insured equipment | item	10