Other Long-Term Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Advance mining royalties	$ 10,910	$ 10,342
Restricted cash	828	794
Other	9,301	7,306
Total other long-term assets	$ 21,039	$ 18,442