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Significant accounting policies - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Significant Accounting Policies [Line Items]    
Net cash provided by operating activities $ 2,079 $ 1,366
Net cash provided by investing activities 1,792 1,534
Accounting Standards Update 2016-15 [Member]    
Significant Accounting Policies [Line Items]    
Net cash provided by operating activities   (962)
Net cash provided by investing activities   $ 962
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member]    
Significant Accounting Policies [Line Items]    
New accounting principle adoption (5)  
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]    
Significant Accounting Policies [Line Items]    
New accounting principle adoption $ 5