Self-Insurance (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) site mi	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Self-Insurance [Abstract]
Number of mining units with self-insured equipment | site	11
Area of self-insured mining units | mi	18
Historical cost of self-insured equipment	$ 267,000	$ 258,000
Restricted cash	$ 4,234	$ 3,811	$ 3,320