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Consolidated net debt - Exchange gains/(losses) on US dollar net debt and intragroup balances (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Net Exchange Gains And Losses [Abstract]      
Exchange gains/(losses) on US dollar net debt $ 554 $ 160 $ (1,578)
Exchange (losses)/gains on intragroup balances (1,154) 449 (1,961)
Exchange (losses)/gains on settlement of dividends (1) 2 1
(Charged)/credited to income statement $ (601) $ 611 $ (3,538)