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Income Tax Expense - Summary of Factors Affecting Income Tax Expense (Parenthetical) (Detail) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2018
Income tax expense benefits [line items]      
Australian prima facie tax rate 30.00% 30.00% 30.00%
US tax reform [member]      
Income tax expense benefits [line items]      
Repatriation tax $ 797 $ 797
Previously unrecognised tax credits $ 603 $ 603