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Summary of Restricted Stock Unit Activity (Detail) - Restricted stock units - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Restricted stock units outstanding at December 31, 2016 2,051,986    
Granted 592,922    
Released (421,108)    
Forfeited (38,334)    
Restricted stock units outstanding at December 31, 2017 2,185,466 2,051,986  
Weighted average grant date fair value      
Restricted stock units outstanding at December 31, 2016 $ 24.12    
Granted 29.94 $ 25.77 $ 28.24
Released 23.24    
Forfeited 27.57    
Restricted stock units outstanding at December 31, 2017 $ 25.81 $ 24.12