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Dividends - Summary of Dividends Paid (Detail) - USD ($)
$ / shares in Units, $ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Jun. 30, 2019
Disclosure of Dividends [abstract]      
Prior year final dividend, Per share $ 0.78 $ 0.63 $ 0.63
Interim dividend, Per share     0.55
Special dividend, Per share     1.02
Dividends paid during the period, Per share $ 0.78 $ 0.63 $ 2.20
Prior year final dividend, Total $ 3,946 $ 3,356 $ 3,356
Interim dividend, Total     2,788
Special dividend, Total     5,158
Dividends paid during the period, Total $ 3,946 $ 3,356 $ 11,302