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Stock Option Plan - Stock Options Issued (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 1,689,000      
Vested (in shares) 990,375      
Exercised (in shares) 546,930      
Forfeited (in shares) 228,750      
Exercisable at December 31 (in shares) 430,945 375,255 435,899  
Remaining unexercised (in shares) 913,320      
Exercise price (in usd per share) | $ / shares $ 46.97 $ 45.59 $ 43.67  
Total value at grant date | $ $ 11,831,000      
Estimated forfeitures | $ 1,914,000      
Expensed in previous years | $ 3,726,000      
Allocated share based compensation | $ 1,448,000 $ 1,348,000 $ 1,176,000  
Future expense | $ 2,219,000      
Directors        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated share based compensation | $ $ 130,700 150,100 143,000  
Directors | 4/25/2008        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 30,000      
Vested (in shares) 30,000      
Exercised (in shares) 20,000      
Forfeited (in shares) 7,500      
Exercisable at December 31 (in shares) 2,500      
Remaining unexercised (in shares) 2,500      
Exercise price (in usd per share) | $ / shares $ 50.15      
Volatility 0.237      
Expected life (years) 7 years      
Assumed yield 4.09%      
Risk-free rate 3.49%      
Total value at grant date | $ $ 255,000      
Expensed in previous years | $ 255,000      
Allocated share based compensation | $ 0 0 0  
Future expense | $ $ 0      
Directors | 4/24/2009        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 32,500      
Vested (in shares) 32,500      
Exercised (in shares) 27,500      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 5,000      
Remaining unexercised (in shares) 5,000      
Exercise price (in usd per share) | $ / shares $ 32.68      
Volatility 0.344      
Expected life (years) 6 years      
Assumed yield 4.54%      
Risk-free rate 2.19%      
Total value at grant date | $ $ 223,000      
Expensed in previous years | $ 223,000      
Allocated share based compensation | $ 0 0 0  
Future expense | $ $ 0      
Directors | 5/7/2010        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 32,500      
Vested (in shares) 32,500      
Exercised (in shares) 25,000      
Forfeited (in shares) 2,500      
Exercisable at December 31 (in shares) 5,000      
Remaining unexercised (in shares) 5,000      
Exercise price (in usd per share) | $ / shares $ 38.76      
Volatility 0.369      
Expected life (years) 5 years      
Assumed yield 4.23%      
Risk-free rate 2.17%      
Total value at grant date | $ $ 288,000      
Expensed in previous years | $ 288,000      
Allocated share based compensation | $ 0 0 0  
Future expense | $ $ 0      
Directors | 5/13/2011        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 32,500      
Vested (in shares) 32,500      
Exercised (in shares) 22,500      
Forfeited (in shares) 2,500      
Exercisable at December 31 (in shares) 7,500      
Remaining unexercised (in shares) 7,500      
Exercise price (in usd per share) | $ / shares $ 41.82      
Volatility 0.358      
Expected life (years) 5 years      
Assumed yield 4.16%      
Risk-free rate 1.86%      
Total value at grant date | $ $ 297,000      
Expensed in previous years | $ 297,000      
Allocated share based compensation | $ 0 0 0  
Future expense | $ $ 0      
Directors | 5/4/2012        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 35,000      
Vested (in shares) 35,000      
Exercised (in shares) 22,500      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 12,500      
Remaining unexercised (in shares) 12,500      
Exercise price (in usd per share) | $ / shares $ 39.29      
Volatility 0.348      
Expected life (years) 5 years      
Assumed yield 4.61%      
Risk-free rate 0.78%      
Total value at grant date | $ $ 257,000      
Expensed in previous years | $ 257,000      
Allocated share based compensation | $ 0 0 0  
Future expense | $ $ 0      
Directors | 5/10/2013        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 35,000      
Vested (in shares) 35,000      
Exercised (in shares) 22,500      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 12,500      
Remaining unexercised (in shares) 12,500      
Exercise price (in usd per share) | $ / shares $ 44.42      
Volatility 0.333      
Expected life (years) 5 years      
Assumed yield 4.53%      
Risk-free rate 0.82%      
Total value at grant date | $ $ 278,000      
Expensed in previous years | $ 278,000      
Allocated share based compensation | $ 0 0 0  
Future expense | $ $ 0      
Directors | 5/9/2014        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 30,000      
Vested (in shares) 30,000      
Exercised (in shares) 17,500      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 12,500      
Remaining unexercised (in shares) 12,500      
Exercise price (in usd per share) | $ / shares $ 47.03      
Volatility 0.173      
Expected life (years) 5 years      
Assumed yield 4.48%      
Risk-free rate 1.63%      
Total value at grant date | $ $ 110,000      
Expensed in previous years | $ 110,000      
Allocated share based compensation | $ 0 0 0  
Future expense | $ $ 0      
Directors | 5/8/2015        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 35,000      
Vested (in shares) 35,000      
Exercised (in shares) 12,500      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 22,500      
Remaining unexercised (in shares) 22,500      
Exercise price (in usd per share) | $ / shares $ 51.07      
Volatility 0.166      
Expected life (years) 5 years      
Assumed yield 4.54%      
Risk-free rate 1.50%      
Total value at grant date | $ $ 125,000      
Expensed in previous years | $ 0      
Allocated share based compensation | $ 0 0 125,000  
Future expense | $ $ 0      
Directors | 5/6/2016        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 32,500      
Vested (in shares) 32,500      
Exercised (in shares) 7,500      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 25,000      
Remaining unexercised (in shares) 25,000      
Exercise price (in usd per share) | $ / shares $ 57.74      
Volatility 0.166      
Expected life (years) 5 years      
Assumed yield 3.75%      
Risk-free rate 1.23%      
Total value at grant date | $ $ 151,000      
Expensed in previous years | $ 0      
Allocated share based compensation | $ 0 151,000 0  
Future expense | $ $ 0      
Directors | 5/5/2017        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 27,500      
Vested (in shares) 27,500      
Exercised (in shares) 0      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 27,500      
Remaining unexercised (in shares) 27,500      
Exercise price (in usd per share) | $ / shares $ 59.41      
Volatility 0.173      
Expected life (years) 5 years      
Assumed yield 3.45%      
Risk-free rate 1.89%      
Total value at grant date | $ $ 166,000      
Expensed in previous years | $ 0      
Allocated share based compensation | $ 166,000 0 0  
Future expense | $ $ 0      
Directors | Subtotals        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 322,500      
Vested (in shares) 322,500      
Exercised (in shares) 177,500      
Forfeited (in shares) 12,500      
Exercisable at December 31 (in shares) 132,500      
Remaining unexercised (in shares) 132,500      
Total value at grant date | $ $ 2,150,000      
Expensed in previous years | $ 1,708,000      
Allocated share based compensation | $ 166,000 151,000 125,000  
Future expense | $ $ 0      
Officers | 5/13/2011        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 162,500      
Vested (in shares) 118,750      
Exercised (in shares) 96,100      
Forfeited (in shares) 43,750      
Exercisable at December 31 (in shares) 22,650      
Remaining unexercised (in shares) 22,650      
Exercise price (in usd per share) | $ / shares $ 41.82      
Volatility 0.330      
Expected life (years) 8 years      
Assumed yield 4.81%      
Risk-free rate 2.75%      
Total value at grant date | $ $ 1,366,000      
Estimated forfeitures | $ 368,000      
Expensed in previous years | $ 909,000      
Allocated share based compensation | $ 0 0 89,000  
Future expense | $ $ 0      
Weighted average term of remaining future expense 2 years 5 months 27 days      
Officers | 5/4/2012        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 242,500      
Vested (in shares) 107,500      
Exercised (in shares) 91,830      
Forfeited (in shares) 135,000      
Exercisable at December 31 (in shares) 15,670      
Remaining unexercised (in shares) 15,670      
Exercise price (in usd per share) | $ / shares $ 39.29      
Volatility 0.315      
Expected life (years) 8 years      
Assumed yield 5.28%      
Risk-free rate 1.49%      
Total value at grant date | $ $ 1,518,000      
Estimated forfeitures | $ 845,000      
Expensed in previous years | $ 419,000      
Allocated share based compensation | $ 0 97,000 157,000  
Future expense | $ $ 0      
Officers | 5/10/2013        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 202,500      
Vested (in shares) 171,875      
Exercised (in shares) 116,500      
Forfeited (in shares) 30,625      
Exercisable at December 31 (in shares) 55,375      
Remaining unexercised (in shares) 55,375      
Exercise price (in usd per share) | $ / shares $ 44.42      
Volatility 0.304      
Expected life (years) 8 years      
Assumed yield 5.12%      
Risk-free rate 1.49%      
Total value at grant date | $ $ 1,401,000      
Estimated forfeitures | $ 212,000      
Expensed in previous years | $ 493,000      
Allocated share based compensation | $ 158,000 269,000 269,000  
Future expense | $ $ 0      
Officers | 5/9/2014        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 170,000      
Vested (in shares) 126,875      
Exercised (in shares) 41,250      
Forfeited (in shares) 1,875      
Exercisable at December 31 (in shares) 85,625      
Remaining unexercised (in shares) 126,875      
Exercise price (in usd per share) | $ / shares $ 47.03      
Volatility 0.306      
Expected life (years) 7 years      
Assumed yield 4.89%      
Risk-free rate 2.17%      
Total value at grant date | $ $ 1,350,000      
Estimated forfeitures | $ 169,000      
Expensed in previous years | $ 197,000      
Allocated share based compensation | $ 295,000 295,000 296,000  
Future expense | $ $ 98,000      
Officers | 5/8/2015        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 190,000      
Vested (in shares) 94,375      
Exercised (in shares) 20,000      
Forfeited (in shares) 3,125      
Exercisable at December 31 (in shares) 74,375      
Remaining unexercised (in shares) 166,875      
Exercise price (in usd per share) | $ / shares $ 51.07      
Volatility 0.298      
Expected life (years) 7 years      
Assumed yield 4.94%      
Risk-free rate 1.89%      
Total value at grant date | $ $ 1,585,000      
Estimated forfeitures | $ 142,000      
Expensed in previous years | $ 0      
Allocated share based compensation | $ 361,000 361,000   $ 240,000
Future expense | $ $ 481,000      
Officers | 5/6/2016        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 194,000      
Vested (in shares) 48,500      
Exercised (in shares) 3,750      
Forfeited (in shares) 1,875      
Exercisable at December 31 (in shares) 44,750      
Remaining unexercised (in shares) 188,375      
Exercise price (in usd per share) | $ / shares $ 57.74      
Volatility 0.185      
Expected life (years) 7 years      
Assumed yield 3.80%      
Risk-free rate 1.55%      
Total value at grant date | $ $ 1,137,000      
Estimated forfeitures | $ 86,000      
Expensed in previous years | $ 0      
Allocated share based compensation | $ 263,000 175,000 0  
Future expense | $ $ 613,000      
Officers | 5/5/2017        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 205,000      
Vested (in shares) 0      
Exercised (in shares) 0      
Forfeited (in shares) 0      
Exercisable at December 31 (in shares) 0      
Remaining unexercised (in shares) 205,000      
Exercise price (in usd per share) | $ / shares $ 59.41      
Volatility 0.170      
Expected life (years) 7 years      
Assumed yield 3.50%      
Risk-free rate 2.17%      
Total value at grant date | $ $ 1,324,000      
Estimated forfeitures | $ 92,000      
Expensed in previous years | $ 0      
Allocated share based compensation | $ 205,000 0 0  
Future expense | $ $ 1,027,000      
Officers | Subtotals        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total grant (in shares) 1,366,500      
Vested (in shares) 667,875      
Exercised (in shares) 369,430      
Forfeited (in shares) 216,250      
Exercisable at December 31 (in shares) 298,445      
Remaining unexercised (in shares) 780,820      
Total value at grant date | $ $ 9,681,000      
Estimated forfeitures | $ 1,914,000      
Expensed in previous years | $ 2,018,000      
Allocated share based compensation | $ 1,282,000 $ 1,197,000 $ 1,051,000  
Future expense | $ $ 2,219,000