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Risk Management Activities - Summary of the Exposure to Credit Risk for Trade Receivables by Geographic Region (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables $ 23.4 $ 46.6
Credit risk [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 23.4 46.6
Credit risk [member] | South Africa [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 0.0 12.8
Credit risk [member] | Ghana [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 8.1 10.4
Credit risk [member] | Australia [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 0.0 2.2
Credit risk [member] | Peru [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables $ 15.3 $ 21.2