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Summary of Nonvested Restricted Stock Unit Awards (Detail) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
WEIGHTED AVERAGE REMAINING VESTING TERM (YEARS)    
Nonvested 1 year 10 months 24 days 1 year 10 months 24 days
Restricted Stock Unit Awards    
NUMBER    
Beginning Balance (in shares) 3,194,899  
Granted (in shares) 970,317  
Vested (in shares) (825,402)  
Forfeited (in shares) (117,595)  
Ending Balance (in shares) 3,222,219 3,194,899
WEIGHTED AVERAGE GRANT-DATE FAIR VALUE PER UNIT    
Nonvested at beginning of year (in dollars per share) $ 53.14  
Granted (in dollars per share) 70.79  
Vested (in dollars per share) 48.31  
Forfeited (in dollars per share) 58.30  
Nonvested at end of year (in dollars per share) $ 59.50 $ 53.14