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Exchange rates in US$ - Summary of principal exchange rate used in preparation of financial statements (Detail)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Sterling      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 1.28 1.28 1.34
Year-end exchange rate per U.S.$ (in dollars per share) 1.36 1.31 1.27
Australian dollar      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 0.69 0.70 0.75
Year-end exchange rate per U.S.$ (in dollars per share) 0.77 0.70 0.70
Canadian dollar      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 0.75 0.75 0.77
Year-end exchange rate per U.S.$ (in dollars per share) 0.78 0.77 0.73
Euro      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 1.14 1.12 1.18
Year-end exchange rate per U.S.$ (in dollars per share) 1.23 1.12 1.14
South African rand      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 0.061 0.069 0.076
Year-end exchange rate per U.S.$ (in dollars per share) 0.068 0.071 0.069