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Supplemental Financial Information - Prepaid expenses and other assets and Other assets (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Prepaid expenses and other assets:    
Prepaid expenses $ 1,592 $ 1,708
Prepaid income taxes 4,772 4,228
Other 1,241 1,896
Other current assets 7,605 7,832
Other long-term assets, net:    
Right of use assets, operating leases, net 4,358 5,073
Spare parts, net 3,360 3,453
Mine development costs, net 7,457 7,084
Mine reclamation asset, net 2,423 2,451
Highview Investment 552 552
Other long-term assets 1,842 1,718
Total $ 19,992 $ 20,331