Stock Option Plans - Stock Options Issued (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,904,000
Vested (in shares)	1,174,500
Exercised (in shares)	566,081
Forfeited (in shares)	231,250
Exercisable at end of period (in shares)	593,419
Remaining unexercised (in shares)	1,106,669
Exercise price (in usd per share) | $ / shares	$ 0.00
Total value at grant date | $	$ 13,059,865
Estimated forfeitures | $	1,727,338
Expensed in previous years | $	9,001,988
Expensed in 2019 | $	396,542
Future expense | $	$ 1,933,997
Weighted average term of remaining future expense (in years)	2 years 4 months 11 days
Directors
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	320,000
Vested (in shares)	320,000
Exercised (in shares)	170,000
Forfeited (in shares)	15,000
Exercisable at end of period (in shares)	135,000
Remaining unexercised (in shares)	135,000
Total value at grant date | $	$ 2,064,650
Expensed in previous years | $	2,064,650
Expensed in 2019 | $	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)	30,000
Forfeited (in shares)	0
Exercisable at end of period (in shares)	2,500
Remaining unexercised (in shares)	2,500
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 2019 | $	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)	27,500
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	2,500
Remaining unexercised (in shares)	2,500
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 2019 | $	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)	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	$ 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 2019 | $	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)	25,000
Forfeited (in shares)	0
Exercisable at end of period (in shares)	10,000
Remaining unexercised (in shares)	10,000
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 2019 | $	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 2019 | $	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 2019 | $	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)	2,500
Exercisable at end of period (in shares)	20,000
Remaining unexercised (in shares)	20,000
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 2019 | $	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)	2,500
Exercisable at end of period (in shares)	22,500
Remaining unexercised (in shares)	22,500
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 2019 | $	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)	5,000
Exercisable at end of period (in shares)	22,500
Remaining unexercised (in shares)	22,500
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 | $	165,550
Expensed in 2019 | $	0
Future expense | $	$ 0
Directors | 5/11/2018
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)	2,500
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	$ 49.46
Volatility (percentage)	0.192
Expected life (years)	5 years
Assumed yield (percentage)	3.70%
Risk-free rate (percentage)	2.84%
Total value at grant date | $	$ 169,400
Expensed in previous years | $	169,400
Expensed in 2019 | $	0
Future expense | $	$ 0
Officers
Share Based Compensation Arrangements By Share Based Payment Award Options [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 end of period (in shares)	458,419
Remaining unexercised (in shares)	971,669
Total value at grant date | $	$ 10,995,215
Estimated forfeitures | $	1,727,338
Expensed in previous years | $	6,937,338
Expensed in 2019 | $	396,542
Future expense | $	$ 1,933,997
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)	103,750
Forfeited (in shares)	43,750
Exercisable at end of period (in shares)	15,000
Remaining unexercised (in shares)	15,000
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 2019 | $	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)	92,455
Forfeited (in shares)	135,000
Exercisable at end of period (in shares)	15,045
Remaining unexercised (in shares)	15,045
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 2019 | $	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)	129,375
Forfeited (in shares)	30,625
Exercisable at end of period (in shares)	42,500
Remaining unexercised (in shares)	42,500
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 | $	1,189,375
Expensed in 2019 | $	0
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)	168,125
Exercised (in shares)	46,126
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	121,999
Remaining unexercised (in shares)	121,999
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 | $	14,887
Expensed in previous years | $	1,334,913
Expensed in 2019 | $	0
Future expense | $	$ 0
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)	140,625
Exercised (in shares)	20,625
Forfeited (in shares)	3,125
Exercisable at end of period (in shares)	120,000
Remaining unexercised (in shares)	166,250
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 | $	26,062
Expensed in previous years | $	1,322,596
Expensed in 2019 | $	176,957
Future expense | $	$ 58,985
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)	96,375
Exercised (in shares)	3,750
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	92,625
Remaining unexercised (in shares)	188,375
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 | $	700,128
Expensed in 2019 | $	65,637
Future expense | $	$ 284,447
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)	51,250
Exercised (in shares)	0
Forfeited (in shares)	0
Exercisable at end of period (in shares)	51,250
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 | $	513,600
Expensed in 2019 | $	77,040
Future expense | $	$ 642,018
Officers | 5/11/2018
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	217,500
Vested (in shares)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	217,500
Exercise price (in usd per share) | $ / shares	$ 49.46
Volatility (percentage)	0.177
Expected life (years)	7 years
Assumed yield (percentage)	3.75%
Risk-free rate (percentage)	2.94%
Total value at grant date | $	$ 1,313,700
Estimated forfeitures | $	83,157
Expensed in previous years | $	205,088
Expensed in 2019 | $	76,908
Future expense | $	$ 948,547