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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, 2018
Dec. 31, 2017
Dec. 31, 2016
United Kingdom, Pounds      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ 1.34 1.29 1.36
Year-end exchange rate per U.S.$ 1.27 1.34 1.22
Australian dollar [Member]      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ 0.75 0.77 0.74
Year-end exchange rate per U.S.$ 0.70 0.78 0.72
Canadian dollar [Member]      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ 0.77 0.77 0.76
Year-end exchange rate per U.S.$ 0.73 0.79 0.74
Euro Member Countries, Euro      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ 1.18 1.13 1.11
Year-end exchange rate per U.S.$ 1.14 1.20 1.05
South African rand [Member]      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ 0.076 0.075 0.068
Year-end exchange rate per U.S.$ 0.069 0.081 0.073