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Cash and cash equivalents - Additional information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of cash and cash equivalents [Line Items]    
Restricted cash and cash equivalents $ 290 $ 358
Repayment of third party borrowings 158 252
Subsidiaries [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Restricted cash and cash equivalents 1,089 1,051
Repayment of third party borrowings $ 703 $ 766