Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Additional Information (Detail)

v3.8.0.1
Stock Based Compensation - Additional Information (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 [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 94,606 95,706 95,706 115,606
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 170,809 258,163 305,520 310,463
Intrinsic value of stock options exercised $ 100,000 $ 0 $ 1,400,000  
Proceeds from stock options exercised $ 100,000 $ 0 $ 1,600,000  
Fair market value of the non-vested stock on the date of grant $ 85.17 $ 89.30 $ 94.74  
Compensation expense-vested portion of non-vested stock grants $ 7,100,000 $ 7,200,000 $ 6,300,000  
Shares granted under performance plan 51,276 23,405 64,665  
Equity Program [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average remaining contractual life of all options (in years) 1 year 10 months 24 days      
Weighted average remaining contractual life of all exercisable options (in years) 1 year 9 months 18 days      
Directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares of non-vested stock issued 3,145      
Deferred compensation expense $ 0 $ 100,000 $ 100,000  
Employees And Directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares of non-vested stock issued 51,276      
Non-vested stock, aggregate fair value $ 4,400,000      
Deferred Compensation Plan For Directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares outstanding under Deferred Compensation Plan 21,540      
Stock Options [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares available for grant for future issuance 345,383      
Shares outstanding under the Plans 76,106      
Common stock authorized for issuance 67,871      
Common shares options outstanding 18,500      
Total unrecognized compensation $ 8,200,000      
Cost recognition, weighted-average period 4 years 2 months 12 days      
Non-vested stock, aggregate fair value $ 5,700,000 $ 4,900,000 $ 4,200,000  
Stock Options [Member] | Equity Program [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation $ 7,000      
Cost recognition, weighted-average period 6 months      
Non-Vested Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares outstanding under the Plans, non-vested shares 3,145      
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 $ 3,000,000      
Cost recognition, weighted-average period 1 year 10 months 24 days      
Shares granted under performance plan 0 0 0  
Compensation expense-Performance awards $ 2,600,000      
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  
Fair market value of the non-vested stock on the date of grant $ 74.36      
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      
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  
Fair market value of the non-vested stock on the date of grant $ 89.07      
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      
Maximum [Member] | Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares granted under performance plan 48,762 37,082 42,538