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Post-retirement benefits - Summary of total amount recognised in other comprehensive income before tax (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [Abstract]      
Actuarial losses to equity accounted units $ (855) $ (1,120) $ 548
Return on assets (net of interest on assets) 894 1,031 79
Loss on application of asset ceiling (33) (1) (8)
Actuarial gains/(losses) on post-retirement benefit plans $ 6 $ (90) $ 619