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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation Roll Forward Analysis Roll Forward    
Beginning balance $ 1,180 $ 1,765
Additions to liabilities 399 105
Reductions to liabilities due to revision of estimates (211) (719)
Accretion expenses 16 29
Ending balance $ 1,384 $ 1,180