XML 444 R234.htm IDEA: XBRL DOCUMENT v3.19.3.a.u2
Exchange rates in US$ - Summary of principal exchange rate used in preparation of financial statements (Detail)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Sterling      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 1.28 1.34 1.29
Year-end exchange rate per U.S.$ (in dollars per share) 1.31 1.27 1.34
Australian dollar      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 0.70 0.75 0.77
Year-end exchange rate per U.S.$ (in dollars per share) 0.70 0.70 0.78
Canadian dollar      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 0.75 0.77 0.77
Year-end exchange rate per U.S.$ (in dollars per share) 0.77 0.73 0.79
Euro      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 1.12 1.18 1.13
Year-end exchange rate per U.S.$ (in dollars per share) 1.12 1.14 1.20
South African rand      
Disclosure Of Foreign Exchange Rates [Line Items]      
Full-year average exchange rate per U.S.$ (in dollars per share) 0.069 0.076 0.075
Year-end exchange rate per U.S.$ (in dollars per share) 0.071 0.069 0.081