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Capital Stock - Additional Information (Detail) - USD ($)
$ / shares in Units, shares in Millions
 9 Months Ended 12 Months Ended 36 Months Ended
Jul. 29, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Dec. 31, 2013
Dec. 31, 2015
Class of Stock [Line Items]            
Cost of shares repurchased   $ 1,787,000,000   $ 3,622,000,000    
Payment for repurchase of common stock   $ 1,727,000,000 $ 3,003,000,000      
Common Class A            
Class of Stock [Line Items]            
Stock repurchase expiration date Dec. 31, 2018       Dec. 31, 2016  
Stock repurchase value $ 13,700,000,000       $ 7,700,000,000  
Increase in share repurchase value $ 6,000,000,000          
Number of shares repurchased   42.9        
Average cost of shares repurchased   $ 41.64        
Cost of shares repurchased           $ 8,200,000,000
Stock repurchase remaining amount   $ 3,700,000,000