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Dividends - Summary of Dividends Paid (Detail) - USD ($)
$ / shares in Units, $ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2018
Disclosure of Dividends [abstract]      
Prior year final dividend, Per share $ 0.63 $ 0.43 $ 0.43
Interim dividend, Per share     0.55
Dividends paid during the period, Per share $ 0.63 $ 0.43 $ 0.98
Prior year final dividend, Total $ 3,356 $ 2,291 $ 2,291
Interim dividend, Total     2,930
Dividends paid during the period, Total $ 3,356 $ 2,291 $ 5,221