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Risk Management Activities - Market Risk - Commodity Price - Oil - Additional Information (Detail)
l in Millions, $ in Millions, $ in Millions
 1 Months Ended 2 Months Ended 12 Months Ended
Jun. 30, 2019
T
ppm
l
bbl
Jun. 30, 2017
ppm
l
bbl
May 31, 2017
T
ppm
l
bbl
Jun. 30, 2017
ppm
l
bbl
Dec. 31, 2020
USD ($)
Dec. 31, 2020
AUD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
AUD ($)
Dec. 31, 2020
AUD ($)
Dec. 31, 2019
AUD ($)
Australia [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member]                    
Disclosure of Financial Risk Management [line items]                    
Marked-to-market value of the hedge         $ (5.1)   $ 1.0   $ (6.6) $ 1.4
Total realized gain/(loss)         (3.4) $ (4.9) 3.1 $ 4.5    
Australia [member] | Commodity price risk [member] | Cash settled swap transaction contracts [member]                    
Disclosure of Financial Risk Management [line items]                    
Unrealized Gain (loss) on hedge ineffectiveness         (5.5) (8.0) (0.8) (1.1)    
Australia [member] | Commodity price risk [member] | Fair value hedges [member]                    
Disclosure of Financial Risk Management [line items]                    
Marked-to-market value of the hedge         27.3   78.3   $ 35.5 $ (111.5)
Total realized gain/(loss)         (201.4) (292.2) (113.4) (163.0)    
Unrealized Gain (loss) on hedge ineffectiveness         71.8 $ 104.0 (65.4) $ (93.4)    
Australia [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member]                    
Disclosure of Financial Risk Management [line items]                    
Risk hedging number | ppm 10 10 10 10            
Volume of diesel hedged | l 75.0     77.5            
Average swap price 74.0 61.2 61.2              
Benchmark price per barrel at time transaction   49.9   49.9            
Australia [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] | Brent crude [member]                    
Disclosure of Financial Risk Management [line items]                    
Benchmark price per barrel at time transaction 57.4 49.9   49.9            
Ghana [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member]                    
Disclosure of Financial Risk Management [line items]                    
Marked-to-market value of the hedge | $         (10.0)   0.1      
Total realized gain/(loss) | $         (6.8)   5.4      
Ghana [member] | Commodity price risk [member] | Cash settled swap transaction contracts [member]                    
Disclosure of Financial Risk Management [line items]                    
Average swap price | T 575   457.2              
Unrealized Gain (loss) on hedge ineffectiveness | $         (10.1)   (2.9)      
Ghana [member] | Commodity price risk [member] | Fair value hedges [member]                    
Disclosure of Financial Risk Management [line items]                    
Marked-to-market value of the hedge | $         0.0   (36.4)      
Total realized gain/(loss) | $         $ (114.5)   $ 2.3      
Ghana [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member]                    
Disclosure of Financial Risk Management [line items]                    
Volume of diesel hedged | l 123.2 125.8 125.8              
Average swap price 75.8 61.4                
Benchmark price per barrel at time transaction   49.8   49.8            
Ghana [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] | Brent crude [member]                    
Disclosure of Financial Risk Management [line items]                    
Benchmark price per barrel at time transaction 59.2 49.8   49.8