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Equity-Accounted Investees - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 06, 2017
Aug. 22, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2010
Disclosure of equity-accounted investees [Line Items]            
Carrying value write down due to loss incurred by the entity     $ 171,300,000 $ 170,700,000 [1]    
Rusoro Mining Limited [Member]            
Disclosure of equity-accounted investees [Line Items]            
Carrying value write down due to loss incurred by the entity           $ 0
Fair value of investment based on quoted market price     7,700,000 23,900,000    
Unrecognised share of profits     2,000,000 18,700,000 $ 3,600,000  
Cumulative share of profits     $ 196,000,000 $ 194,000,000    
Pre and post award   $ 967,800,000        
Excess of pre and post award   $ 1,200,000,000        
Arbitration settlement $ 1,300,000,000          
Accrued interest percentage     9.00%      
[1] As Restated - Refer note 40 for further details.