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Stock Option Plans - Stock Options Issued (Details)	3 Months Ended	9 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares	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)	554,580
Forfeited (in shares)	228,750
Exercisable at end of period (in shares)	423,295
Remaining unexercised (in shares)	905,670
Exercise price (in usd per share) | $ / shares	$ 41.82
Total value at grant date | $	$ 11,831,465
Estimated forfeitures | $	1,759,899
Expensed in previous years | $	7,698,535
Expensed in 2018 | $	422,068
Future expense | $	$ 1,950,963
Weighted average term of remaining future expense (in years)	2 years 3 months 20 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 | $	2,149,950
Expensed in 2018 | $	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)		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 2018 | $		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 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
Officers
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)
Vested (in shares)
Exercised (in shares)
Forfeited (in shares)
Exercisable at end of period (in shares)
Remaining unexercised (in shares)
Total value at grant date | $
Estimated forfeitures | $
Expensed in previous years | $
Expensed in 2018 | $
Future expense | $
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)	126,875
Exercised (in shares)	41,250
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	85,625
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 | $	189,214
Future expense | $	$ 63,035
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)	20,000
Forfeited (in shares)	3,125
Exercisable at end of period (in shares)	74,375
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 | $	90,177
Future expense | $	$ 390,755
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)	3,750
Forfeited (in shares)	1,875
Exercisable at end of period (in shares)	44,750
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 | $	65,637
Future expense | $	$ 546,995
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 | $	205,440
Expensed in 2018 | $	77,040
Future expense | $	$ 950,178
Officers | 5/3/2018
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)		377,080
Forfeited (in shares)		216,250
Exercisable at end of period (in shares)		290,795
Remaining unexercised (in shares)		773,170
Total value at grant date | $		$ 9,681,515
Estimated forfeitures | $		1,759,899
Expensed in previous years | $		5,548,585
Expensed in 2018 | $		422,068
Future expense | $		$ 1,950,963

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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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