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Stock Option Plan - Stock Options Issued (Detail) | 12 Months Ended
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Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total grant (in shares) | 1,904,000 |  |
Vested (in shares) | 1,174,500 |  |
Exercised (in shares) | 566,081 |  |
Forfeited (in shares) | 223,750 |  |
Exercisable at December 31 (in shares) | 600,919 | 430,945 | 375,255
Remaining unexercised (in shares) | 1,114,169 |  |
Exercise price (in usd per share) | $ / shares | $ 42.98 | $ 46.97 | $ 45.59
Total value at grant date | $ | $ 13,060,000 |  |
Estimated forfeitures | $ | 1,843,000 |  |
Expensed in previous years | $ | 4,648,000 |  |
Allocated share based compensation | $ | 1,558,000 | $ 1,448,000 | $ 1,348,000
Future expense | $ | 2,215,000 |  |
Directors |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Allocated share based compensation | $ | $ 108,800 | 130,700 | 150,100
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) | 30,000 |  |
Forfeited (in shares) | 0 |  |
Exercisable at December 31 (in shares) | 2,500 |  |
Remaining unexercised (in shares) | 2,500 |  |
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) | 27,500 |  |
Forfeited (in shares) | 2,500 |  |
Exercisable at December 31 (in shares) | 2,500 |  |
Remaining unexercised (in shares) | 2,500 |  |
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) | 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 | $ 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) | 25,000 |  |
Forfeited (in shares) | 0 |  |
Exercisable at December 31 (in shares) | 10,000 |  |
Remaining unexercised (in shares) | 10,000 |  |
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 |  |
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 | $ | 125,000 |  |
Allocated share based compensation | $ | 0 | 0 | 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) | 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 | 0 | 151,000
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) | 2,500 |  |
Exercisable at December 31 (in shares) | 25,000 |  |
Remaining unexercised (in shares) | 25,000 |  |
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 | $ | 0 | 166,000 | 0
Future expense | $ | $ 0 |  |
Directors | 5/11/2018 |  |  |
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) | 2,500 |  |
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 | $ 49.46 |  |
Volatility | 0.192 |  |
Expected life (years) | 5 years |  |
Assumed yield | 3.70% |  |
Risk-free rate | 2.84% |  |
Total value at grant date | $ | $ 169,000 |  |
Expensed in previous years | $ | 0 |  |
Allocated share based compensation | $ | 169,000 | 0 | 0
Future expense | $ | $ 0 |  |
Directors | Subtotals |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total grant (in shares) | 320,000 |  |
Vested (in shares) | 320,000 |  |
Exercised (in shares) | 170,000 |  |
Forfeited (in shares) | 7,500 |  |
Exercisable at December 31 (in shares) | 142,500 |  |
Remaining unexercised (in shares) | 142,500 |  |
Total value at grant date | $ | $ 2,065,000 |  |
Expensed in previous years | $ | 1,579,000 |  |
Allocated share based compensation | $ | 169,000 | 166,000 | 151,000
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) | 103,750 |  |
Forfeited (in shares) | 43,750 |  |
Exercisable at December 31 (in shares) | 15,000 |  |
Remaining unexercised (in shares) | 15,000 |  |
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 | $ | 999,000 |  |
Allocated share based compensation | $ | 0 | 0 | 0
Future expense | $ | $ 0 |  |
Weighted average term of remaining future expense | 2 years 6 months 11 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) | 92,455 |  |
Forfeited (in shares) | 135,000 |  |
Exercisable at December 31 (in shares) | 15,045 |  |
Remaining unexercised (in shares) | 15,045 |  |
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 | $ | 576,000 |  |
Allocated share based compensation | $ | 0 | 0 | 97,000
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) | 129,375 |  |
Forfeited (in shares) | 30,625 |  |
Exercisable at December 31 (in shares) | 42,500 |  |
Remaining unexercised (in shares) | 42,500 |  |
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 | $ | 762,000 |  |
Allocated share based compensation | $ | 0 | 158,000 | 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) | 168,125 |  |
Exercised (in shares) | 46,126 |  |
Forfeited (in shares) | 1,875 |  |
Exercisable at December 31 (in shares) | 121,999 |  |
Remaining unexercised (in shares) | 121,999 |  |
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 | $ | 15,000 |  |
Expensed in previous years | $ | 492,000 |  |
Allocated share based compensation | $ | 252,000 | 295,000 | 296,000
Future expense | $ | $ 0 |  |
Officers | 5/8/2015 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total grant (in shares) | 190,000 |  |
Vested (in shares) | 140,625 |  |
Exercised (in shares) | 20,625 |  |
Forfeited (in shares) | 3,125 |  |
Exercisable at December 31 (in shares) | 120,000 |  |
Remaining unexercised (in shares) | 166,250 |  |
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 | $ | 240,000 |  |
Allocated share based compensation | $ | 361,000 | 361,000 | 360,000
Future expense | $ | $ 121,000 |  |
Officers | 5/6/2016 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total grant (in shares) | 194,000 |  |
Vested (in shares) | 96,375 |  |
Exercised (in shares) | 3,750 |  |
Forfeited (in shares) | 1,875 |  |
Exercisable at December 31 (in shares) | 92,625 |  |
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 | 263,000 | 175,000
Future expense | $ | $ 350,000 |  |
Officers | 5/5/2017 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total grant (in shares) | 205,000 |  |
Vested (in shares) | 51,250 |  |
Exercised (in shares) | 0 |  |
Forfeited (in shares) | 0 |  |
Exercisable at December 31 (in shares) | 51,250 |  |
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 | $ | 308,000 | 205,000 | 0
Future expense | $ | $ 719,000 |  |
Officers | 5/11/2018 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total grant (in shares) | 217,500 |  |
Vested (in shares) | 0 |  |
Exercised (in shares) | 0 |  |
Forfeited (in shares) | 0 |  |
Exercisable at December 31 (in shares) | 0 |  |
Remaining unexercised (in shares) | 217,500 |  |
Exercise price (in usd per share) | $ / shares | $ 49.46 |  |
Volatility | 0.177 |  |
Expected life (years) | 7 years |  |
Assumed yield | 3.75% |  |
Risk-free rate | 2.94% |  |
Total value at grant date | $ | $ 1,313,000 |  |
Estimated forfeitures | $ | 83,000 |  |
Expensed in previous years | $ | 0 |  |
Allocated share based compensation | $ | 205,000 | 0 | 0
Future expense | $ | $ 1,025,000 |  |
Officers | Subtotals |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total grant (in shares) | 1,584,000 |  |
Vested (in shares) | 854,500 |  |
Exercised (in shares) | 396,081 |  |
Forfeited (in shares) | 216,250 |  |
Exercisable at December 31 (in shares) | 458,419 |  |
Remaining unexercised (in shares) | 971,669 |  |
Total value at grant date | $ | $ 10,995,000 |  |
Estimated forfeitures | $ | 1,843,000 |  |
Expensed in previous years | $ | 3,069,000 |  |
Allocated share based compensation | $ | 1,389,000 | $ 1,282,000 | $ 1,197,000
Future expense | $ | $ 2,215,000 |  |

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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which option holders acquired shares when converting their stock options into shares. + References No definition available. + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=115993241&loc=d3e301413-122809 + Details
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