Principal subsidiaries - Summary of subsidiaries financial information (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Significant Investments In Subsidiaries [Line Items]
Goodwill	$ 912	$ 1,037	$ 951
North Mining Limited [member] \| Robe river iron associates [member]
Disclosure Of Significant Investments In Subsidiaries [Line Items]
Goodwill	$ 351	$ 389

The amount of assets representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognised. [Refer: Business combinations [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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