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Risk Management Activities - Market Risk - Commodity Price - Oil - Additional Information (Detail) - Fair value hedges [member]
l in Millions, $ in Millions, $ in Millions
 1 Months Ended 12 Months Ended
Jun. 30, 2019
ppm
l
$ / bbl
$ / Tonne
Jun. 30, 2017
ppm
l
$ / bbl
$ / Barrel
$ / Tonne
May 31, 2017
ppm
l
$ / bbl
$ / Barrel
$ / Tonne
Dec. 31, 2019
USD ($)
Dec. 31, 2019
AUD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
AUD ($)
Dec. 31, 2019
AUD ($)
Dec. 31, 2018
AUD ($)
Australia [member] | Cash settled swap transaction contracts [member]                  
Disclosure Of Financial Risk Management [line items]                  
Marked-to-market value of the hedge       $ 1.0   $ 1.7   $ 1.4 $ 2.5
Total realized gain       3.1 $ 4.5 4.6 $ 6.1    
Australia [member] | Commodity price risk [member]                  
Disclosure Of Financial Risk Management [line items]                  
Marked-to-market value of the hedge       (78.3)   12.3   $ (111.5) $ 17.6
Total realized gain       (113.4) $ (163.0) 8.4 $ 11.2    
Australia [member] | Commodity price risk [member] | Cash settled swap transaction contracts [member]                  
Disclosure Of Financial Risk Management [line items]                  
Risk hedging number | ppm 10 10 10            
Volume of diesel hedged | l 75.0 77.5 77.5            
Average swap price | $ / bbl 74.0 61.2 61.2            
Australia [member] | Commodity price risk [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] | Cash settled swap transaction contracts [member]                  
Disclosure Of Financial Risk Management [line items]                  
Marked-to-market value of the hedge       0.1   3.0      
Total realized gain       5.4   7.5      
Ghana [member] | Commodity price risk [member]                  
Disclosure Of Financial Risk Management [line items]                  
Marked-to-market value of the hedge       (36.4)   2.4      
Total realized gain       $ 2.3   $ 19.6      
Ghana [member] | Commodity price risk [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 | $ / bbl 75.8 61.4 61.4            
Average swap price | $ / Tonne 575 457.2 457.2            
Ghana [member] | Commodity price risk [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