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Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - Silvertip Mine - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at the start of the period $ 49,635 $ 49,276
Revaluation 0 0
Settlements 0 0
Accretion 182 541
Balance at the end of the period $ 49,817 $ 49,817