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Risk Management Activities - Market Risk - Foreign Currency - Additional Information (Detail)
$ in Millions
 1 Months Ended 12 Months Ended 30 Months Ended
Dec. 31, 2018
USD ($)
Oct. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
May 31, 2018
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
AUD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
AUD ($)
Dec. 31, 2022
CLP ($)
Dec. 31, 2019
AUD ($)
Dec. 31, 2018
AUD ($)
Disclosure Of Financial Risk Management [line items]                        
Forwards notional value for the period January 2019 to December 2019 $ 1,290,000,000         $ 1,200,000,000   $ 1,290,000,000        
Average strike rate                   836.45    
Currency risk [member]                        
Disclosure Of Financial Risk Management [line items]                        
Exposure to risk relating to financial instruments           0            
Average strike rate 71.5 70.75 71.82 73.3 75.17              
Currency risk [member] | Forward contract [member]                        
Disclosure Of Financial Risk Management [line items]                        
Forwards notional value for the period January 2019 to December 2019 $ 50,000,000 $ 60,000,000 $ 100,000,000 $ 60,000,000 $ 96,000,000     50,000,000        
Mark to Market value $ 8,700,000         300,000   8,700,000     $ 0.4 $ 12.3
Gains (losses) on ineffectiveness of hedges of net investments in foreign operations recognised in profit or loss           $ (15,500,000) $ (22.3) $ 0 $ 0.0