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Acquisitions and disposals - Additional information (Detail) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Disclosure Of Acquisitions And Disposals [Line Items]    
Disposals of subsidiaries, joint ventures, unincorporated joint operations and associates [1]   $ 46
Yancoal    
Disclosure Of Acquisitions And Disposals [Line Items]    
Disposals of subsidiaries, joint ventures, unincorporated joint operations and associates [1] $ 10  
Royalty proceeds   20
Hail Creek Coal Mine    
Disclosure Of Acquisitions And Disposals [Line Items]    
Cash received from working capital adjustments   $ 26
[1] Refer to Acquisitions and disposals note on page F-14.