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Capital Stock - Summary of Normal Course Issuer Bids (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Schedule Of Share Repurchase Programs [Line Items]    
Shares purchased 10,752,484 35,069,804
Cash amount $ 501 $ 1,766
2013 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 40,000,000  
Shares purchased   30,271,428
Cash amount   $ 1,525
2014 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 40,000,000  
Shares purchased 8,166,514 4,798,376
Cash amount $ 388 $ 241
2015 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Maximum number of shares 40,000,000  
Shares purchased 2,585,970  
Cash amount $ 113