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Risk Management Activities - Schedule of Combined Maximum Credit Risk Exposure (Detail) - Credit risk [member] - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Environmental trust funds [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk $ 69.5 $ 60.8
Trade and other receivable [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk [1] 74.2 64.9 [2]
Cash and cash equivalent [member]    
Disclosure of credit risk exposure [line items]    
Maximum exposure to credit risk $ 515.0 $ 219.7
[1] Trade and other receivables above exclude VAT, import duties, prepayments, payroll receivables, derivative contracts and diesel rebates amounting to US$62.9 million (2018: US$73.7 million).
[2] Refer note 42.