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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, 2019
Dec. 31, 2018
 [1]
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables $ 23.6 $ 15.3
Credit risk [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 23.6 15.3
Credit risk [member] | Ghana [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 0.8  
Credit risk [member] | Peru [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables $ 22.8 $ 15.3
[1] Refer note 42.