Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Additional Information (Detail)

v3.10.0.1
Stock Based Compensation - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized for issuance under the Plan       561,000
Stock option plan, share based compensation, vesting period (in years) 1 year      
Share-based compensation exercise period       10 years
Shares outstanding under the Plans 23,000 94,606 95,706 95,706
Number of shares for initial granting of options 3,500      
Number of shares for annual granting of options 2,000      
Percentage of annual fees in the form of non-vested shares 80.00%      
Common stock authorized for issuance 150,000      
Shares outstanding under the Plans, non-vested shares 96,669 170,809 258,163 305,520
Intrinsic value of stock options exercised $ 3,500,000 $ 100,000 $ 0  
Proceeds from stock options exercised $ 3,100,000 $ 100,000    
Fair market value of the non-vested stock on the date of grant $ 95.32 $ 85.17 $ 89.30  
Compensation expense-vested portion of non-vested stock grants $ 6,000,000 $ 7,100,000 $ 7,200,000  
Shares granted under performance plan 31,879 51,276 23,405  
Equity Program [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average remaining contractual life of all exercisable options (in years) 5 years 6 months      
Directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares of non-vested stock issued 4,183      
Deferred compensation expense $ 0 $ 0 $ 100,000  
Deferred Compensation Plan For Directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares outstanding under Deferred Compensation Plan 22,520      
Stock Options [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares available for grant for future issuance 290,659      
Shares outstanding under the Plans 4,500      
Common stock authorized for issuance 63,688      
Common shares options outstanding 18,500      
Non-vested stock, aggregate fair value $ 4,700,000 $ 5,700,000 $ 4,900,000  
Total unrecognized compensation $ 7,200,000      
Cost recognition, weighted-average period 3 years 10 months 24 days      
Non-Vested Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares outstanding under the Plans, non-vested shares 4,183      
Non-Vested Shares [Member] | Employees And Directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares of non-vested stock issued 31,879      
Aggregate fair value $ 3,000,000      
Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option plan, share based compensation, vesting period (in years) 3 years  
Total unrecognized compensation $ 2,600,000      
Cost recognition, weighted-average period 2 years 4 months 24 days      
Shares granted under performance plan 0 0 0  
Compensation expense-Performance awards $ 2,100,000      
Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option plan, share based compensation, vesting period (in years)       4 years
Minimum [Member] | Non-Vested Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option plan, share based compensation, vesting period (in years) 1 year      
Fair market value of the non-vested stock on the date of grant $ 81.86      
Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option plan, share based compensation, vesting period (in years)       8 years
Maximum [Member] | Non-Vested Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option plan, share based compensation, vesting period (in years) 9 years      
Fair market value of the non-vested stock on the date of grant $ 99.71      
Maximum [Member] | Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares granted under performance plan 34,760 48,762 37,082