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Taxation - Summary of tax relating to components of other comprehensive income or loss (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Relating To Components Of Other Comprehensive Income That Will Be Reclassified To Profit Or Loss [Abstract]      
Deferred tax charge included in other comprehensive income $ 153 $ 33 $ 174
Current tax charge included in other comprehensive income   $ 0 $ 4