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Stock Option Plan - Stock Options Issued (Detail)
 12 Months Ended
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
Dec. 31, 2013
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 1,425,000    
Vested (in shares) | shares 724,375    
Exercised (in shares) | shares 265,976    
Forfeited (in shares) | shares 298,750    
Exercisable at December 31 (in shares) | shares 435,899 380,708 413,000
Remaining unexercised (in shares) | shares 860,274    
Exercise price (in usd per share) | $ / shares $ 43.67 $ 37.71 $ 33.15
Total value at grant date $ 10,822,000    
Estimated forfeitures 1,911,000    
Expensed in previous years 3,588,000    
Future expense 2,302,000    
Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 880,000    
Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 965,000    
Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 1,176,000    
Directors      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 143,000 $ 112,900 $ 124,400
Directors | 5/1/2006      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 30,000    
Vested (in shares) | shares 30,000    
Exercised (in shares) | shares 20,000    
Forfeited (in shares) | shares 2,500    
Exercisable at December 31 (in shares) | shares 7,500    
Remaining unexercised (in shares) | shares 7,500    
Exercise price (in usd per share) | $ / shares $ 40.35    
Volatility 0.206    
Expected life (years) 9 years    
Assumed yield 5.93%    
Risk-free rate 5.11%    
Total value at grant date $ 144,000    
Expensed in previous years 144,000    
Future expense 0    
Directors | 5/1/2006 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/1/2006 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/1/2006 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 4/27/2007      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 30,000    
Vested (in shares) | shares 30,000    
Exercised (in shares) | shares 5,000    
Forfeited (in shares) | shares 7,500    
Exercisable at December 31 (in shares) | shares 17,500    
Remaining unexercised (in shares) | shares 17,500    
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    
Future expense 0    
Directors | 4/27/2007 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 4/27/2007 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 4/27/2007 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 4/25/2008      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 30,000    
Vested (in shares) | shares 30,000    
Exercised (in shares) | shares 7,500    
Forfeited (in shares) | shares 7,500    
Exercisable at December 31 (in shares) | shares 15,000    
Remaining unexercised (in shares) | shares 15,000    
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    
Future expense 0    
Directors | 4/25/2008 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 4/25/2008 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 4/25/2008 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 4/24/2009      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 32,500    
Vested (in shares) | shares 32,500    
Exercised (in shares) | shares 22,500    
Forfeited (in shares) | shares 0    
Exercisable at December 31 (in shares) | shares 10,000    
Remaining unexercised (in shares) | shares 10,000    
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    
Future expense 0    
Directors | 4/24/2009 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 4/24/2009 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 4/24/2009 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 5/7/2010      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 32,500    
Vested (in shares) | shares 32,500    
Exercised (in shares) | shares 15,000    
Forfeited (in shares) | shares 2,500    
Exercisable at December 31 (in shares) | shares 15,000    
Remaining unexercised (in shares) | shares 15,000    
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    
Future expense 0    
Directors | 5/7/2010 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/7/2010 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/7/2010 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 5/13/2011      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 32,500    
Vested (in shares) | shares 32,500    
Exercised (in shares) | shares 15,000    
Forfeited (in shares) | shares 2,500    
Exercisable at December 31 (in shares) | shares 15,000    
Remaining unexercised (in shares) | shares 15,000    
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    
Future expense 0    
Directors | 5/13/2011 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/13/2011 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/13/2011 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 5/4/2012      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 35,000    
Vested (in shares) | shares 35,000    
Exercised (in shares) | shares 15,000    
Forfeited (in shares) | shares 0    
Exercisable at December 31 (in shares) | shares 20,000    
Remaining unexercised (in shares) | shares 20,000    
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    
Future expense 0    
Directors | 5/4/2012 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/4/2012 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/4/2012 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 5/10/2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 35,000    
Vested (in shares) | shares 35,000    
Exercised (in shares) | shares 12,500    
Forfeited (in shares) | shares 0    
Exercisable at December 31 (in shares) | shares 22,500    
Remaining unexercised (in shares) | shares 22,500    
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 0    
Future expense 0    
Directors | 5/10/2013 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 278,000    
Directors | 5/10/2013 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/10/2013 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 5/9/2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 30,000    
Vested (in shares) | shares 30,000    
Exercised (in shares) | shares 7,500    
Forfeited (in shares) | shares 0    
Exercisable at December 31 (in shares) | shares 22,500    
Remaining unexercised (in shares) | shares 22,500    
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 0    
Future expense 0    
Directors | 5/9/2014 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/9/2014 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 110,000    
Directors | 5/9/2014 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Directors | 5/8/2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 35,000    
Vested (in shares) | shares 35,000    
Exercised (in shares) | shares 5,000    
Forfeited (in shares) | shares 0    
Exercisable at December 31 (in shares) | shares 30,000    
Remaining unexercised (in shares) | 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    
Future expense 0    
Directors | 5/8/2015 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/8/2015 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Directors | 5/8/2015 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 125,000    
Directors | Subtotals      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 322,500    
Vested (in shares) | shares 322,500    
Exercised (in shares) | shares 125,000    
Forfeited (in shares) | shares 22,500    
Exercisable at December 31 (in shares) | shares 175,000    
Remaining unexercised (in shares) | shares 175,000    
Total value at grant date $ 2,262,000    
Expensed in previous years 1,749,000    
Future expense 0    
Directors | Subtotals | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 278,000    
Directors | Subtotals | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 110,000    
Directors | Subtotals | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 125,000    
Officers | 4/27/2007      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 135,000    
Vested (in shares) | shares 67,500    
Exercised (in shares) | shares 14,097    
Forfeited (in shares) | shares 67,500    
Exercisable at December 31 (in shares) | shares 53,403    
Remaining unexercised (in shares) | shares 53,403    
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    
Future expense $ 0    
Weighted average term of remaining future expense 2 years 7 months 25 days    
Officers | 4/27/2007 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Officers | 4/27/2007 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Officers | 4/27/2007 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 0    
Officers | 5/13/2011      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 162,500    
Vested (in shares) | shares 118,750    
Exercised (in shares) | shares 58,754    
Forfeited (in shares) | shares 43,750    
Exercisable at December 31 (in shares) | shares 59,996    
Remaining unexercised (in shares) | shares 59,996    
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 $ 1,367,000    
Estimated forfeitures 368,000    
Expensed in previous years 457,000    
Future expense 0    
Officers | 5/13/2011 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 236,000    
Officers | 5/13/2011 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 217,000    
Officers | 5/13/2011 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 89,000    
Officers | 5/4/2012      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 242,500    
Vested (in shares) | shares 81,875    
Exercised (in shares) | shares 40,625    
Forfeited (in shares) | shares 135,000    
Exercisable at December 31 (in shares) | shares 41,250    
Remaining unexercised (in shares) | shares 66,875    
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 890,000    
Expensed in previous years 105,000    
Future expense 52,000    
Officers | 5/4/2012 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 157,000    
Officers | 5/4/2012 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 157,000    
Officers | 5/4/2012 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 157,000    
Officers | 5/10/2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 202,500    
Vested (in shares) | shares 91,250    
Exercised (in shares) | shares 24,375    
Forfeited (in shares) | shares 30,000    
Exercisable at December 31 (in shares) | shares 66,875    
Remaining unexercised (in shares) | shares 148,125    
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 0    
Future expense 359,000    
Officers | 5/10/2013 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 209,000    
Officers | 5/10/2013 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 284,000    
Officers | 5/10/2013 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 269,000    
Officers | 5/9/2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 170,000    
Vested (in shares) | shares 42,500    
Exercised (in shares) | shares 3,125    
Forfeited (in shares) | shares 0    
Exercisable at December 31 (in shares) | shares 39,375    
Remaining unexercised (in shares) | shares 166,875    
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 0    
Future expense 689,000    
Officers | 5/9/2014 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Officers | 5/9/2014 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 197,000    
Officers | 5/9/2014 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 295,000    
Officers | 5/8/2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 190,000    
Vested (in shares) | shares 0    
Exercised (in shares) | shares 0    
Forfeited (in shares) | shares 0    
Exercisable at December 31 (in shares) | shares 0    
Remaining unexercised (in shares) | shares 190,000    
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    
Future expense 1,202,000    
Officers | 5/8/2015 | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Officers | 5/8/2015 | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 0    
Officers | 5/8/2015 | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 241,000    
Officers | Subtotals      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant (in shares) | shares 1,102,500    
Vested (in shares) | shares 401,875    
Exercised (in shares) | shares 140,976    
Forfeited (in shares) | shares 276,250    
Exercisable at December 31 (in shares) | shares 260,899    
Remaining unexercised (in shares) | shares 685,274    
Total value at grant date $ 8,560,000    
Estimated forfeitures 1,911,000    
Expensed in previous years 1,839,000    
Future expense 2,302,000    
Officers | Subtotals | Expensed in 2013      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 602,000    
Officers | Subtotals | Expensed in 2014      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation 855,000    
Officers | Subtotals | Expensed in 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share based compensation $ 1,051,000