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Capital Stock - Summary of Normal Course Issuer Bids (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Share Repurchase Programs [Line Items]    
Shares purchased 26,257,043 22,335,018
Cash amount $ 1,271 $ 904
2015 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 40,000,000  
Shares purchased   20,313,194
Cash amount   $ 818
2016 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 38,000,000  
Shares purchased 23,245,377 2,021,824
Cash amount $ 1,109 $ 86
2017 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 35,800,000  
Shares purchased 3,011,666  
Cash amount $ 162