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Stock Option Plan - Stock Options Issued (Detail)
 12 Months Ended
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) 1,621,500    
Vested (in shares) 883,125    
Exercised (in shares) 487,870    
Forfeited (in shares) 300,000    
Exercisable at December 31 (in shares) 375,255 435,899 380,708
Remaining unexercised (in shares) 833,630    
Exercise price (in usd per share) | $ / shares $ 45.59 $ 43.67 $ 37.71
Total value at grant date | $ $ 11,966,000    
Estimated forfeitures | $ 1,953,000    
Expensed in previous years | $ 4,324,000    
Allocated share based compensation | $ 1,348,000 $ 1,176,000 $ 965,000
Future expense | $ 2,200,000    
Directors      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation | $ $ 150,100 143,000 112,900
Directors | 4/27/2007      
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) 10,000    
Forfeited (in shares) 7,500    
Exercisable at December 31 (in shares) 12,500    
Remaining unexercised (in shares) 12,500    
Exercise price (in usd per share) | $ / shares $ 54.17    
Volatility 0.225    
Expected life (years) 8 years    
Assumed yield 4.39%    
Risk-free rate 4.65%    
Total value at grant date | $ $ 285,000    
Expensed in previous years | $ 285,000    
Allocated share based compensation | $ 0 0 0
Future expense | $ $ 0    
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) 12,500    
Forfeited (in shares) 7,500    
Exercisable at December 31 (in shares) 10,000    
Remaining unexercised (in shares) 10,000    
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) 25,000    
Forfeited (in shares) 0    
Exercisable at December 31 (in shares) 7,500    
Remaining unexercised (in shares) 7,500    
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) 17,500    
Forfeited (in shares) 2,500    
Exercisable at December 31 (in shares) 12,500    
Remaining unexercised (in shares) 12,500    
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) 17,500    
Forfeited (in shares) 2,500    
Exercisable at December 31 (in shares) 12,500    
Remaining unexercised (in shares) 12,500    
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 | $ $ 298,000    
Expensed in previous years | $ 298,000    
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) 17,500    
Forfeited (in shares) 0    
Exercisable at December 31 (in shares) 17,500    
Remaining unexercised (in shares) 17,500    
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) 15,000    
Forfeited (in shares) 0    
Exercisable at December 31 (in shares) 20,000    
Remaining unexercised (in shares) 20,000    
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) 10,000    
Forfeited (in shares) 0    
Exercisable at December 31 (in shares) 20,000    
Remaining unexercised (in shares) 20,000    
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 | $ 0    
Allocated share based compensation | $ 0 0 110,000
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) 5,000    
Forfeited (in shares) 0    
Exercisable at December 31 (in shares) 30,000    
Remaining unexercised (in shares) 30,000    
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 | $ 0 125,000 0
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) 0    
Forfeited (in shares) 0    
Exercisable at December 31 (in shares) 32,500    
Remaining unexercised (in shares) 32,500    
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 | $ 151,000 0 0
Future expense | $ $ 0    
Directors | Subtotals      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) 325,000    
Vested (in shares) 325,000    
Exercised (in shares) 130,000    
Forfeited (in shares) 20,000    
Exercisable at December 31 (in shares) 175,000    
Remaining unexercised (in shares) 175,000    
Total value at grant date | $ $ 2,270,000    
Expensed in previous years | $ 1,884,000    
Allocated share based compensation | $ 151,000 125,000 110,000
Future expense | $ $ 0    
Officers | 4/27/2007      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) 135,000    
Vested (in shares) 67,500    
Exercised (in shares) 67,500    
Forfeited (in shares) 67,500    
Exercisable at December 31 (in shares) 0    
Remaining unexercised (in shares) 0    
Exercise price (in usd per share) | $ / shares $ 54.17    
Volatility 0.233    
Expected life (years) 6 years 6 months    
Assumed yield 4.13%    
Risk-free rate 4.61%    
Total value at grant date | $ $ 1,339,000    
Estimated forfeitures | $ 62,000    
Expensed in previous years | $ 1,277,000    
Allocated share based compensation | $ 0    
Future expense | $ $ 0    
Weighted average term of remaining future expense 2 years 5 months 20 days    
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) 92,915    
Forfeited (in shares) 43,750    
Exercisable at December 31 (in shares) 25,835    
Remaining unexercised (in shares) 25,835    
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 | $ 692,000    
Allocated share based compensation | $ 0 89,000 217,000
Future expense | $ $ 0    
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) 91,205    
Forfeited (in shares) 135,000    
Exercisable at December 31 (in shares) 16,295    
Remaining unexercised (in shares) 16,295    
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 | $ 262,000    
Allocated share based compensation | $ 97,000 157,000 157,000
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) 131,875    
Exercised (in shares) 68,750    
Forfeited (in shares) 30,625    
Exercisable at December 31 (in shares) 63,125    
Remaining unexercised (in shares) 103,125    
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 | $ 280,000    
Expensed in previous years | $ 209,000    
Allocated share based compensation | $ 269,000 269,000 284,000
Future expense | $ $ 90,000    
Officers | 5/9/2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) 170,000    
Vested (in shares) 85,000    
Exercised (in shares) 31,250    
Forfeited (in shares) 1,250    
Exercisable at December 31 (in shares) 53,750    
Remaining unexercised (in shares) 137,500    
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 | $ 0    
Allocated share based compensation | $ 295,000 295,000 197,000
Future expense | $ $ 394,000    
Officers | 5/8/2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) 190,000    
Vested (in shares) 47,500    
Exercised (in shares) 6,250    
Forfeited (in shares) 1,875    
Exercisable at December 31 (in shares) 41,250    
Remaining unexercised (in shares) 181,875    
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 | $ 361,000 241,000 0
Future expense | $ $ 841,000    
Officers | 5/6/2016      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) 194,000    
Vested (in shares) 0    
Exercised (in shares) 0    
Forfeited (in shares) 0    
Exercisable at December 31 (in shares) 0    
Remaining unexercised (in shares) 194,000    
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 | $ 87,000    
Expensed in previous years | $ 0    
Allocated share based compensation | $ 175,000 0 0
Future expense | $ $ 875,000    
Officers | Subtotals      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) 1,296,500    
Vested (in shares) 558,125    
Exercised (in shares) 357,870    
Forfeited (in shares) 280,000    
Exercisable at December 31 (in shares) 200,255    
Remaining unexercised (in shares) 658,630    
Total value at grant date | $ $ 9,696,000    
Estimated forfeitures | $ 1,953,000    
Expensed in previous years | $ 2,440,000    
Allocated share based compensation | $ 1,197,000 $ 1,051,000 $ 855,000
Future expense | $ $ 2,200,000