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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [Abstract]      
Actuarial gains/(losses) $ (1,295) $ 1,382 $ (855)
Return on assets (net of interest on assets) 1,033 (527) 894
Gain/(loss) on application of asset ceiling 0 52 (33)
Total gain/(loss) recognised in other comprehensive income $ (262) $ 907 $ 6