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Risk Management Activities - Schedule of Contractually Due Undiscounted Cash Flows Resulting from Maturities of All Financial Liabilities, Including Interest Payments (Parenthetical) (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
Exchange_Rates
Dec. 31, 2019
USD ($)
Exchange_Rates
Dec. 31, 2018
USD ($)
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Funding from environmental trust funds $ 79.3 $ 69.5 $ 60.8
Notional amount 1,200.0    
South Africa and Ghana [member]      
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Funding from environmental trust funds $ 79.3 69.5  
US $1 Billion Notes [member]      
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Borrowings interest rate description 4.875    
Notional amount $ 1,000.0 1,000.0  
US$500 million 5-year notes issue (the 5-year notes) [member]      
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Borrowings interest rate description 5.125    
Notional amount   500.0  
US$500 million 10-year notes issue (the 10-year notes) [member]      
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Borrowings interest rate description 6.125    
Notional amount $ 500.0 $ 500.0  
Closing foreign exchange rate [member]      
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Foreign exchange rate | Exchange_Rates 14.69 14.00  
Liquidity Risk [Member] | US dollar borrowings [member]      
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Borrowings interest rate description   Spot LIBOR (one month fix) rate adjusted by specific facility agreement: 0.14388% (2019: 1.7625%  
Borrowings adjustment to interest rate 0.14388% 1.7625%  
Liquidity Risk [Member] | Australian Dollar borrowings [member]      
Disclosure of maturity analysis for non-derivative financial liabilities [line items]      
Borrowings interest rate description Spot Bank Bill Swap Bid Rate (BBSY) (one month fix) rate adjusted by specific facility agreement: 0.06% (2019: 0.92%)    
Borrowings adjustment to interest rate 0.06% 0.92%