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Subsidiaries - Summary of Significant Subsidiaries (Parenthetical) (Detail) - BHP Iron Ore (Jimblebar) Pty Ltd [member]
12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Disclosure of significant subsidiaries [Line Items]    
Effective Group interest 92.50% 92.50%
Group interest 85.00% 85.00%