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Equity-Accounted Investees - Summary of Equity-Accounted Investees (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of equity-accounted investees [Line Items]      
Total equity-accounted investees $ 42.7 $ 42.1  
Total equity-accounted investees 171.3 170.7 [1]  
Total share of results of equity-accounted investees after taxation (1.3) (2.3) [1] $ (5.7) [1]
Far Southeast Gold Resources Incorporated [Member]      
Disclosure of equity-accounted investees [Line Items]      
Total equity-accounted investees 128.6 128.6  
Total share of results of equity-accounted investees after taxation (1.6) (2.3) (3.3)
Marverix Metals Incorporated [Member]      
Disclosure of equity-accounted investees [Line Items]      
Total equity-accounted investees 42.7 42.1  
Total share of results of equity-accounted investees after taxation 0.3 0.0 0.0
Other [Member]      
Disclosure of equity-accounted investees [Line Items]      
Total equity-accounted investees 0.0 0.0  
Total share of results of equity-accounted investees after taxation $ 0.0 $ 0.0 $ (2.4)
[1] As Restated - Refer note 40 for further details.