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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [Abstract]      
Actuarial (losses)/gains $ (1,242) $ (1,295) $ 1,382
Return on assets, net of interest on assets 768 1,033 (527)
Gain on application of asset ceiling 0 0 52
Total (loss)/gain recognised in other comprehensive income $ (474) $ (262) $ 907