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Supplemental Financial Information - Prepaid expenses and other assets and Other assets (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Prepaid expenses and other assets:    
Prepaid expenses $ 2,370 $ 1,690
Prepaid income taxes and income tax refunds 1,038 1,605
Other 1,206 1,302
Other current assets 4,614 4,597
Other long-term assets, net:    
Cabot receivable 6,812 8,852
Upfront Customer Consideration 7,109 7,490
Mine development costs, net 5,291 4,338
Right of use assets, operating leases, net 6,399 1,930
Spare parts, net 4,316 3,727
Mine reclamation asset, net 1,644 1,712
Highview Investment 552 552
Other 1,278 1,388
Total $ 33,401 $ 29,989