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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Dec. 31, 2014
Asset Retirement Obligation Roll Forward Analysis Roll Forward      
Beginning balance $ 1,180 $ 1,765 $ 204
Additions to liabilities   105 1,544
Reductions to liabilities due to revision of estimates   (719)  
Accretion expenses 54 29 17
Ending balance $ 1,234 $ 1,180 $ 1,765