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Stock Option Plans - Stock Options Issued (Details)	6 Months Ended
	Jun. 30, 2018 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)	534,580
Forfeited (in shares)	221,250
Exercisable at end of period (in shares)	634,920
Remaining unexercised (in shares)	1,148,170
Exercise price (in usd per share) | $ / shares	$ 41.82
Total value at grant date | $	$ 13,059,865
Estimated forfeitures | $	1,843,056
Expensed in previous years | $	7,443,835
Expensed in 2018 | $	938,629
Future expense | $	$ 2,834,345
Weighted average term of remaining future expense (in years)	2 years 10 months 12 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)	157,500
Forfeited (in shares)	5,000
Exercisable at end of period (in shares)	157,500
Remaining unexercised (in shares)	157,500
Total value at grant date | $	$ 2,064,650
Expensed in previous years | $	1,895,250
Expensed in 2018 | $	169,400
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 2018 | $	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 2018 | $	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 2018 | $	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 2018 | $	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 2018 | $	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 2018 | $	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 2018 | $	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 2018 | $	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 | $	165,550
Expensed in 2018 | $	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)	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	$ 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 | $	0
Expensed in 2018 | $	169,400
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)	377,080
Forfeited (in shares)	216,250
Exercisable at end of period (in shares)	477,420
Remaining unexercised (in shares)	990,670
Total value at grant date | $	$ 10,995,215
Estimated forfeitures | $	1,843,056
Expensed in previous years | $	5,548,585
Expensed in 2018 | $	769,229
Future expense | $	$ 2,834,345
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 2018 | $	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 2018 | $	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)	116,500
Forfeited (in shares)	30,625
Exercisable at end of period (in shares)	55,375
Remaining unexercised (in shares)	55,375
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 2018 | $	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)	41,250
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	126,875
Remaining unexercised (in shares)	126,875
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 | $	1,082,664
Expensed in 2018 | $	252,249
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)	20,000
Forfeited (in shares)	3,125
Exercisable at end of period (in shares)	120,625
Remaining unexercised (in shares)	166,875
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 | $	961,888
Expensed in 2018 | $	180,354
Future expense | $	$ 300,578
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 | $	437,580
Expensed in 2018 | $	131,274
Future expense | $	$ 481,358
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 | $	205,440
Expensed in 2018 | $	154,080
Future expense | $	$ 873,138
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 | $	0
Expensed in 2018 | $	51,272
Future expense | $	$ 1,179,271

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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