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Exceptional Items - Additional Information (Detail) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Jun. 30, 2019
Disclosure of exceptional items [line items]      
Exceptional gain (loss), before tax $ 784 $ 210 $ 1,060
Samarco dam failure [member]      
Disclosure of exceptional items [line items]      
Exceptional gain (loss), before tax $ 6 [1] $ 210 $ 1,060
[1] Refer to note 9 Significant events – Samarco dam failure for further information.