Stock Option Plans - Stock Options Issued (Details)	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,689,000
Vested (in shares)	990,375
Exercised (in shares)	493,036
Forfeited (in shares)	228,750
Exercisable at end of period (in shares)	484,839
Remaining unexercised (in shares)	967,214
Exercise price (in usd per share) | $ / shares	$ 47.03
Total value at grant date | $	$ 11,831,465
Estimated forfeitures | $	1,913,761
Expensed in previous years | $	6,250,776
Expensed in 2017 | $	1,141,087
Future expense | $	$ 2,525,841
Weighted average term of remaining future expense (in years)	2 years 7 months 27 days
Directors
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	322,500
Vested (in shares)	322,500
Exercised (in shares)	177,500
Forfeited (in shares)	12,500
Exercisable at end of period (in shares)	132,500
Remaining unexercised (in shares)	132,500
Total value at grant date | $	$ 2,149,950
Expensed in previous years | $	1,984,400
Expensed in 2017 | $	165,550
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)	20,000
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	2,500
Remaining unexercised (in shares)	2,500
Exercise price (in usd per share) | $ / shares	$ 50.15
Volatility (percentage)	0.237
Expected life (years)	7 years
Assumed yield (percentage)	4.09%
Risk-free rate (percentage)	3.49%
Total value at grant date | $	$ 254,700
Expensed in previous years | $	254,700
Expensed in 2017 | $	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)	27,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	5,000
Remaining unexercised (in shares)	5,000
Exercise price (in usd per share) | $ / shares	$ 32.68
Volatility (percentage)	0.344
Expected life (years)	6 years
Assumed yield (percentage)	4.54%
Risk-free rate (percentage)	2.19%
Total value at grant date | $	$ 222,950
Expensed in previous years | $	222,950
Expensed in 2017 | $	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)	25,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	5,000
Remaining unexercised (in shares)	5,000
Exercise price (in usd per share) | $ / shares	$ 38.76
Volatility (percentage)	0.369
Expected life (years)	5 years
Assumed yield (percentage)	4.23%
Risk-free rate (percentage)	2.17%
Total value at grant date | $	$ 287,950
Expensed in previous years | $	287,950
Expensed in 2017 | $	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)	22,500
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	7,500
Remaining unexercised (in shares)	7,500
Exercise price (in usd per share) | $ / shares	$ 41.82
Volatility (percentage)	0.358
Expected life (years)	5 years
Assumed yield (percentage)	4.16%
Risk-free rate (percentage)	1.86%
Total value at grant date | $	$ 297,375
Expensed in previous years | $	297,375
Expensed in 2017 | $	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)	22,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	12,500
Remaining unexercised (in shares)	12,500
Exercise price (in usd per share) | $ / shares	$ 39.29
Volatility (percentage)	0.348
Expected life (years)	5 years
Assumed yield (percentage)	4.61%
Risk-free rate (percentage)	0.78%
Total value at grant date | $	$ 257,250
Expensed in previous years | $	257,250
Expensed in 2017 | $	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)	22,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	12,500
Remaining unexercised (in shares)	12,500
Exercise price (in usd per share) | $ / shares	$ 44.42
Volatility (percentage)	0.333
Expected life (years)	5 years
Assumed yield (percentage)	4.53%
Risk-free rate (percentage)	0.82%
Total value at grant date | $	$ 278,250
Expensed in previous years | $	278,250
Expensed in 2017 | $	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)	17,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	12,500
Remaining unexercised (in shares)	12,500
Exercise price (in usd per share) | $ / shares	$ 47.03
Volatility (percentage)	0.173
Expected life (years)	5 years
Assumed yield (percentage)	4.48%
Risk-free rate (percentage)	1.63%
Total value at grant date | $	$ 109,500
Expensed in previous years | $	109,500
Expensed in 2017 | $	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)	12,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	22,500
Remaining unexercised (in shares)	22,500
Exercise price (in usd per share) | $ / shares	$ 51.07
Volatility (percentage)	0.166
Expected life (years)	5 years
Assumed yield (percentage)	4.54%
Risk-free rate (percentage)	1.50%
Total value at grant date | $	$ 125,300
Expensed in previous years | $	125,300
Expensed in 2017 | $	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)	7,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	25,000
Remaining unexercised (in shares)	25,000
Exercise price (in usd per share) | $ / shares	$ 57.74
Volatility (percentage)	0.166
Expected life (years)	5 years
Assumed yield (percentage)	3.75%
Risk-free rate (percentage)	1.23%
Total value at grant date | $	$ 151,125
Expensed in previous years | $	151,125
Expensed in 2017 | $	0
Future expense | $	$ 0
Directors | 5/5/2017
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	27,500
Vested (in shares)	27,500
Exercised (in shares)	0
Forfeited (in shares)	0
Exercisable at end of period (in shares)	27,500
Remaining unexercised (in shares)	27,500
Exercise price (in usd per share) | $ / shares	$ 59.41
Volatility (percentage)	0.173
Expected life (years)	5 years
Assumed yield (percentage)	3.45%
Risk-free rate (percentage)	1.89%
Total value at grant date | $	$ 165,550
Expensed in previous years | $	0
Expensed in 2017 | $	165,550
Future expense | $	$ 0
Officers
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,366,500
Vested (in shares)	667,875
Exercised (in shares)	315,536
Forfeited (in shares)	216,250
Exercisable at end of period (in shares)	352,339
Remaining unexercised (in shares)	834,714
Total value at grant date | $	$ 9,681,515
Estimated forfeitures | $	1,913,761
Expensed in previous years | $	4,266,376
Expensed in 2017 | $	975,537
Future expense | $	$ 2,525,841
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)	96,100
Forfeited (in shares)	43,750
Exercisable at end of period (in shares)	22,650
Remaining unexercised (in shares)	22,650
Exercise price (in usd per share) | $ / shares	$ 41.82
Volatility (percentage)	0.330
Expected life (years)	8 years
Assumed yield (percentage)	4.81%
Risk-free rate (percentage)	2.75%
Total value at grant date | $	$ 1,366,625
Estimated forfeitures | $	367,937
Expensed in previous years | $	998,688
Expensed in 2017 | $	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)	91,830
Forfeited (in shares)	135,000
Exercisable at end of period (in shares)	15,670
Remaining unexercised (in shares)	15,670
Exercise price (in usd per share) | $ / shares	$ 39.29
Volatility (percentage)	0.315
Expected life (years)	8 years
Assumed yield (percentage)	5.28%
Risk-free rate (percentage)	1.49%
Total value at grant date | $	$ 1,518,050
Estimated forfeitures | $	845,100
Expensed in previous years | $	672,950
Expensed in 2017 | $	0
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)	171,875
Exercised (in shares)	83,000
Forfeited (in shares)	30,625
Exercisable at end of period (in shares)	88,875
Remaining unexercised (in shares)	88,875
Exercise price (in usd per share) | $ / shares	$ 44.42
Volatility (percentage)	0.304
Expected life (years)	8 years
Assumed yield (percentage)	5.12%
Risk-free rate (percentage)	1.49%
Total value at grant date | $	$ 1,401,300
Estimated forfeitures | $	211,925
Expensed in previous years | $	1,031,134
Expensed in 2017 | $	158,241
Future expense | $	$ 0
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)	126,875
Exercised (in shares)	35,000
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	91,875
Remaining unexercised (in shares)	133,125
Exercise price (in usd per share) | $ / shares	$ 47.03
Volatility (percentage)	0.306
Expected life (years)	7 years
Assumed yield (percentage)	4.89%
Risk-free rate (percentage)	2.17%
Total value at grant date | $	$ 1,349,800
Estimated forfeitures | $	168,749
Expensed in previous years | $	787,392
Expensed in 2017 | $	221,454
Future expense | $	$ 172,205
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)	94,375
Exercised (in shares)	8,981
Forfeited (in shares)	3,125
Exercisable at end of period (in shares)	85,394
Remaining unexercised (in shares)	177,894
Exercise price (in usd per share) | $ / shares	$ 51.07
Volatility (percentage)	0.298
Expected life (years)	7 years
Assumed yield (percentage)	4.94%
Risk-free rate (percentage)	1.89%
Total value at grant date | $	$ 1,584,600
Estimated forfeitures | $	141,780
Expensed in previous years | $	601,180
Expensed in 2017 | $	270,531
Future expense | $	$ 571,109
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)	48,500
Exercised (in shares)	625
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	47,875
Remaining unexercised (in shares)	191,500
Exercise price (in usd per share) | $ / shares	$ 57.74
Volatility (percentage)	0.185
Expected life (years)	7 years
Assumed yield (percentage)	3.80%
Risk-free rate (percentage)	1.55%
Total value at grant date | $	$ 1,136,840
Estimated forfeitures | $	86,628
Expensed in previous years | $	175,032
Expensed in 2017 | $	196,911
Future expense | $	$ 678,269
Officers | 5/5/2017
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	205,000
Vested (in shares)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	205,000
Exercise price (in usd per share) | $ / shares	$ 59.41
Volatility (percentage)	0.170
Expected life (years)	7 years
Assumed yield (percentage)	3.50%
Risk-free rate (percentage)	2.17%
Total value at grant date | $	$ 1,324,300
Estimated forfeitures | $	91,642
Expensed in previous years | $	0
Expensed in 2017 | $	128,400
Future expense | $	$ 1,104,258