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Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2019
USD ($)
Available-for-sale Securities  
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Balance at the start of the period $ 2,217
Revaluation 59
Settlements (2,276)
Accretion 0
Balance at the end of the period 0
Silvertip Mine  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Balance at the start of the period 49,276
Revaluation 0
Settlements 0
Accretion 179
Balance at the end of the period $ 49,455