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Reversal of Impairment (Narrative) (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2013
Disclosure of information for impairment loss recognised or reversed for individual asset or cash-generating unit [line items]        
Reversal of impairment loss $ 31,119      
Impairment loss 2,637 $ 1,423    
Caylloma Plant and Mine Equipment [member] | Individual assets or cash-generating units [member]        
Disclosure of information for impairment loss recognised or reversed for individual asset or cash-generating unit [line items]        
Reversal of impairment loss $ 31,119      
Impairment loss     $ 55,000 $ 55,000
Discount rate applied to cash flow projections 4.10%