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Interest in associates and joint ventures - Associates Commentary (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Disclosure of associates [line items]        
Carrying amount of interests in associates $ 22,577,000,000   $ 19,874,000,000  
Bank of Communications Co., Limited        
Disclosure of associates [line items]        
Carrying amount of interests in associates $ 18,057,000,000 $ 18,057,000,000 $ 15,765,000,000 $ 15,765,000,000
Investment's fair value below carrying amount, period 68 months      
Impairment loss $ 0