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Stock-Based Compensation - Details of Nonvested Restricted Stock Units Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
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, Weighted Average Grant Date Fair Value [Abstract]      
Granted (in dollars per share) $ 0.00 $ 0.00  
Granted (in dollars per share) $ 0.00 $ 0.00  
Weighted average grant-date fair value of shares vested $ 1,400 $ 2,633 $ 4,602
Restricted stock units      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Beginning Balance (in shares) 240,574    
Granted (in shares) 103,207    
Vested (in shares) (157,344)    
Canceled (in shares) (8,479)    
Accrued dividend equivalents (in shares) 2,430    
Ending Balance (in shares) 180,388 240,574  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Beginning Balance (in dollars per share) $ 35.75    
Granted (in dollars per share) 38.33 $ 36.02 $ 36.35
Vested (in dollars per share) 35.76    
Canceled (in dollars per share) 37.76    
Accrued dividend equivalents (in dollars per share) 36.70    
Ending Balance (in dollars per share) 37.14 35.75  
Granted (in dollars per share) $ 38.33 $ 36.02 $ 36.35
Weighted average grant-date fair value of shares vested $ 5,627 $ 3,195 $ 2,629