Annual report pursuant to Section 13 and 15(d)

Stock Option and Purchase Plans - Narrative (Detail)

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Stock Option and Purchase Plans - Narrative (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award        
Period of options granted (in years) 5 years      
Share price (in usd per share) $ 41.47 $ 33.84 $ 35.40  
Weighted average fair value of options vested (in usd per share) $ 14.25 $ 12.05 $ 10.85  
Total fair value of options that vested during the year $ 1,600,000 $ 1,400,000 $ 1,500,000  
Total compensation costs related to non-vested awards $ 4,800,000      
Weighted average period (in years) 2 years 4 months      
Options outstanding (in shares) 1,310,090 1,338,273 1,444,954 1,686,178
Weighted average fair value of options granted (in usd per share) $ 17.60 $ 16.85 $ 18.00  
Employee Stock        
Share-based Compensation Arrangement by Share-based Payment Award        
Share price (in usd per share) $ 25.63      
Cash compensation limit $ 21,250      
Common stock price to market value (percentage) 85.00%      
Number of shares employees had subscribed to purchase (in shares) 129,798      
Weighted average fair value of options granted (in usd per share) $ 5.92 $ 9.88 $ 6.93  
Stock Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Stock option exercisable period (not exceeding, in years) 10 years      
Period of options granted (in years) 5 years      
Options outstanding (in shares) 1,034,898      
Directors Stock Option Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Stock option exercisable period (not exceeding, in years) 10 years      
Period of options granted (in years) 6 months      
Options outstanding (in shares) 204,052      
Long Term Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Options outstanding (in shares) 71,140      
Options available for future grant (in shares) 1,685,899      
Director        
Share-based Compensation Arrangement by Share-based Payment Award        
Period of options granted (in years) 6 months      
Key Employee        
Share-based Compensation Arrangement by Share-based Payment Award        
Period of options granted (in years) 5 years      
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award        
Stock option exercisable period (not exceeding, in years) 10 years