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Exceptional Items - Additional Information (Detail) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2018
Disclosure of exceptional items [line items]      
Exceptional items net $ (32) $ 2,530 $ 2,970
Samarco dam failure [member]      
Disclosure of exceptional items [line items]      
Exceptional items net $ 210 $ 210 $ 650