Supplemental Financial Information - Other Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Other current liabilities:
Current portion of operating lease obligations	$ 2,025	$ 2,382
Accrued interest	116	213
Income and other taxes payable	708	678
Other	528	1,038
Other current liabilities	3,377	4,311
Other long-term liabilities:
Operating lease obligations, long-term	1,954	2,810
Mine reclamation liability	2,804	2,721
Other	58	229
Other long-term liabilities	$ 4,816	$ 5,760
Mine reserves, amortization period	21 years