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Stock Option Plan - Stock Options Issued (Detail) | 12 Months Ended
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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 |  |

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- Definition Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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