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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, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [Abstract]      
Actuarial gains/(losses) $ 655 $ (1,242) $ (1,295)
Return on assets, net of interest on assets 371 768 1,033
Re-measurement gains/(losses) on pension and post-retirement healthcare plans $ 1,026 $ (474) $ (262)