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Capital Stock - Summary of Normal Course Issuer Bids (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Schedule Of Share Repurchase Programs [Line Items]    
Shares purchased 22,335,018 10,752,484
Cash amount $ 904 $ 501
2014 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 40,000,000  
Shares purchased   8,166,514
Cash amount   $ 388
2015 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 40,000,000  
Shares purchased 20,313,194 2,585,970
Cash amount $ 818 $ 113
2016 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 38,000,000  
Shares purchased 2,021,824  
Cash amount $ 86