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Stock Option Plans - Stock Options Issued (Details)
 6 Months Ended
Jun. 30, 2016
USD ($)
$ / shares
shares
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 1,621,500
Vested (in shares) 883,125
Exercised (in shares) 410,708
Forfeited (in shares) 296,250
Exercisable at end of period (in shares) 452,417
Remaining unexercised (in shares) 914,542
Exercise price (in usd per share) | $ / shares $ 45.76
Total value at grant date | $ $ 11,966,115
Estimated forfeitures | $ 1,952,662
Expensed in previous years | $ 6,465,083
Expensed in 2016 | $ 754,379
Future expense | $ $ 2,793,991
Weighted average term of remaining future expense (in years) 2 years 10 months 2 days
Directors  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 325,000
Vested (in shares) 325,000
Exercised (in shares) 125,000
Forfeited (in shares) 20,000
Exercisable at end of period (in shares) 180,000
Remaining unexercised (in shares) 180,000
Total value at grant date | $ $ 2,269,700
Expensed in previous years | $ 2,118,575
Expensed in 2016 | $ 151,125
Future expense | $ $ 0
Directors | 4/27/2007  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 30,000
Vested (in shares) 30,000
Exercised (in shares) 7,500
Forfeited (in shares) 7,500
Exercisable at end of period (in shares) 15,000
Remaining unexercised (in shares) 15,000
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 | $ $ 285,300
Expensed in previous years | $ 285,300
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 4/25/2008  
Share Based Compensation Arrangements By Share Based Payment Award Options [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 end of period (in shares) 12,500
Remaining unexercised (in shares) 12,500
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 | $ $ 254,700
Expensed in previous years | $ 254,700
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 4/24/2009  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 32,500
Vested (in shares) 32,500
Exercised (in shares) 25,000
Forfeited (in shares) 0
Exercisable at end of period (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 | $ $ 222,950
Expensed in previous years | $ 222,950
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 5/7/2010  
Share Based Compensation Arrangements By Share Based Payment Award Options [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 end of period (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 | $ $ 287,950
Expensed in previous years | $ 287,950
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 5/13/2011  
Share Based Compensation Arrangements By Share Based Payment Award Options [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 end of period (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 | $ $ 297,375
Expensed in previous years | $ 297,375
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 5/4/2012  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 35,000
Vested (in shares) 35,000
Exercised (in shares) 17,500
Forfeited (in shares) 0
Exercisable at end of period (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 | $ $ 257,250
Expensed in previous years | $ 257,250
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 5/10/2013  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 35,000
Vested (in shares) 35,000
Exercised (in shares) 15,000
Forfeited (in shares) 0
Exercisable at end of period (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 | $ $ 278,250
Expensed in previous years | $ 278,250
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 5/9/2014  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 30,000
Vested (in shares) 30,000
Exercised (in shares) 10,000
Forfeited (in shares) 0
Exercisable at end of period (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 | $ $ 109,500
Expensed in previous years | $ 109,500
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 5/8/2015  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 35,000
Vested (in shares) 35,000
Exercised (in shares) 5,000
Forfeited (in shares) 0
Exercisable at end of period (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 | $ $ 125,300
Expensed in previous years | $ 125,300
Expensed in 2016 | $ 0
Future expense | $ $ 0
Directors | 5/6/2016  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 32,500
Vested (in shares) 32,500
Exercised (in shares) 0
Forfeited (in shares) 0
Exercisable at end of period (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 | $ $ 151,125
Expensed in previous years | $ 0
Expensed in 2016 | $ 151,125
Future expense | $ $ 0
Officers  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 1,296,500
Vested (in shares) 558,125
Exercised (in shares) 285,708
Forfeited (in shares) 276,250
Exercisable at end of period (in shares) 272,417
Remaining unexercised (in shares) 734,542
Total value at grant date | $ $ 9,696,415
Estimated forfeitures | $ 1,952,662
Expensed in previous years | $ 4,346,508
Expensed in 2016 | $ 603,254
Future expense | $ $ 2,793,991
Officers | 4/27/2007  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 135,000
Vested (in shares) 67,500
Exercised (in shares) 55,168
Forfeited (in shares) 67,500
Exercisable at end of period (in shares) 12,332
Remaining unexercised (in shares) 12,332
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 | $ $ 1,339,200
Estimated forfeitures | $ 62,000
Expensed in previous years | $ 1,277,200
Expensed in 2016 | $ 0
Future expense | $ $ 0
Officers | 5/13/2011  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 162,500
Vested (in shares) 118,750
Exercised (in shares) 84,915
Forfeited (in shares) 43,750
Exercisable at end of period (in shares) 33,835
Remaining unexercised (in shares) 33,835
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 | $ $ 1,366,625
Estimated forfeitures | $ 367,937
Expensed in previous years | $ 998,688
Expensed in 2016 | $ 0
Future expense | $ $ 0
Officers | 5/4/2012  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 242,500
Vested (in shares) 107,500
Exercised (in shares) 67,500
Forfeited (in shares) 135,000
Exercisable at end of period (in shares) 40,000
Remaining unexercised (in shares) 40,000
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 | $ $ 1,518,050
Estimated forfeitures | $ 845,100
Expensed in previous years | $ 575,994
Expensed in 2016 | $ 96,956
Future expense | $ $ 0
Officers | 5/10/2013  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 202,500
Vested (in shares) 131,875
Exercised (in shares) 55,625
Forfeited (in shares) 30,000
Exercisable at end of period (in shares) 76,250
Remaining unexercised (in shares) 116,875
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 | $ $ 1,401,300
Estimated forfeitures | $ 280,468
Expensed in previous years | $ 762,034
Expensed in 2016 | $ 134,550
Future expense | $ $ 224,248
Officers | 5/9/2014  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 170,000
Vested (in shares) 85,000
Exercised (in shares) 17,500
Forfeited (in shares) 0
Exercisable at end of period (in shares) 67,500
Remaining unexercised (in shares) 152,500
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 | $ $ 1,349,800
Estimated forfeitures | $ 168,749
Expensed in previous years | $ 492,120
Expensed in 2016 | $ 147,636
Future expense | $ $ 541,295
Officers | 5/8/2015  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 190,000
Vested (in shares) 47,500
Exercised (in shares) 5,000
Forfeited (in shares) 0
Exercisable at end of period (in shares) 42,500
Remaining unexercised (in shares) 185,000
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 | $ $ 1,584,600
Estimated forfeitures | $ 141,780
Expensed in previous years | $ 240,472
Expensed in 2016 | $ 180,354
Future expense | $ $ 1,021,994
Officers | 5/6/2016  
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]  
Total grant (in shares) 194,000
Vested (in shares) 0
Exercised (in shares) 0
Forfeited (in shares) 0
Exercisable at end of period (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 | $ $ 1,136,840
Estimated forfeitures | $ 86,628
Expensed in previous years | $ 0
Expensed in 2016 | $ 43,758
Future expense | $ $ 1,006,454