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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, 2020
Dec. 31, 2019
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables $ 40.7 $ 23.6
Credit risk [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 40.7 23.6
Credit risk [member] | South Africa [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 0.0 0.0
Credit risk [member] | Ghana [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 14.3 0.8
Credit risk [member] | Australia [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables 2.7 0.0
Credit risk [member] | Peru [member]    
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]    
Total trade receivables $ 23.7 $ 22.8