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Risk Management Activities - Schedule of Combined Maximum Credit Risk Exposure (Detail) - Credit risk [member] - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Environmental trust funds [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk $ 79.3 $ 69.5
Trade and other receivable [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk [1] 53.2 74.2
Loan advanced - contractor [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk 68.4 0.0
Derivative financial assets [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk 113.3 1.1
Cash and cash equivalent [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk $ 886.8 $ 515.0
[1] Trade and other receivables above exclude VAT, import duties, prepayments, payroll receivables and diesel rebates amounting to US$186.9 million (2019: US$61.8 million).