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Stock Option Plans - Stock Options Issued (Details)	3 Months Ended
	Mar. 31, 2017 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)	504,120
Forfeited (in shares)	300,000
Exercisable at end of period (in shares)	359,005
Remaining unexercised (in shares)	817,380
Exercise price (in usd per share) | $ / shares	$ 49.52
Total value at grant date | $	$ 11,966,115
Estimated forfeitures | $	1,884,119
Expensed in previous years | $	7,813,276
Expensed in 2017 | $	296,910
Future expense | $	$ 1,971,810
Weighted average term of remaining future expense (in years)	2 years 2 months 27 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)	145,000
Forfeited (in shares)	20,000
Exercisable at end of period (in shares)	160,000
Remaining unexercised (in shares)	160,000
Total value at grant date | $	$ 2,269,700
Expensed in previous years | $	2,269,700
Expensed in 2017 | $	0
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)	17,500
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	5,000
Remaining unexercised (in shares)	5,000
Exercise price (in usd per share) | $ / shares	$ 54.17
Volatility (percentage)	0.225
Expected life (years)	8 years
Assumed yield (percentage)	4.39%
Risk-free rate (percentage)	4.65%
Total value at grant date | $	$ 285,300
Expensed in previous years | $	285,300
Expensed in 2017 | $	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)	17,500
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	5,000
Remaining unexercised (in shares)	5,000
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)	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 (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)	20,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	10,000
Remaining unexercised (in shares)	10,000
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)	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 (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)	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 (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)	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 (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)	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 (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)	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 (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)	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 (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
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)	359,120
Forfeited (in shares)	280,000
Exercisable at end of period (in shares)	199,005
Remaining unexercised (in shares)	657,380
Total value at grant date | $	$ 9,696,415
Estimated forfeitures | $	1,884,119
Expensed in previous years | $	5,543,576
Expensed in 2017 | $	296,910
Future expense | $	$ 1,971,810
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)	67,500
Forfeited (in shares)	67,500
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	0
Exercise price (in usd per share) | $ / shares	$ 54.17
Volatility (percentage)	0.233
Expected life (years)	6 years 6 months
Assumed yield (percentage)	4.13%
Risk-free rate (percentage)	4.61%
Total value at grant date | $	$ 1,339,200
Estimated forfeitures | $	62,000
Expensed in previous years | $	1,277,200
Expensed in 2017 | $	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)	93,540
Forfeited (in shares)	43,750
Exercisable at end of period (in shares)	25,210
Remaining unexercised (in shares)	25,210
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)	131,875
Exercised (in shares)	68,750
Forfeited (in shares)	30,625
Exercisable at end of period (in shares)	63,125
Remaining unexercised (in shares)	103,125
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 | $	67,278
Future expense | $	$ 90,963
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)	31,250
Forfeited (in shares)	1,250
Exercisable at end of period (in shares)	53,750
Remaining unexercised (in shares)	137,500
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 | $	73,818
Future expense | $	$ 319,841
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)	6,250
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	41,250
Remaining unexercised (in shares)	181,875
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 | $	601,180
Expensed in 2017 | $	90,177
Future expense | $	$ 751,463
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 (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 | $	65,637
Future expense | $	$ 809,543

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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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