Stock Option Plans - Stock Options Issued (Details)	9 Months Ended
	Sep. 30, 2015 USD ($) $ / shares shares
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Exercisable at end of period (in shares) | shares	476,524
Exercise price (in usd per share) | $ / shares	$ 42.71
Weighted average term of remaining future expense (in years)	2 years 10 months 6 days
Directors | 5/1/2006
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 01, 2006
Total grant (in shares) | shares	30,000
Vested (in shares)	$ 30,000
Exercised (in shares) | shares	15,000
Forfeited (in shares) | shares	2,500
Exercisable at end of period (in shares) | shares	12,500
Remaining unexercised (in shares) | shares	12,500
Exercise price (in usd per share) | $ / shares	$ 40.35
Volatility	0.206
Expected life (years)	9 years
Assumed yield	5.93%
Risk-free rate	5.11%
Total value at grant date	$ 143,400
Expensed in previous years	143,400
Expensed in 2015	0
Future expense	$ 0
Directors | 4/27/2007
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 27, 2007
Total grant (in shares) | shares	30,000
Vested (in shares)	$ 30,000
Exercised (in shares) | shares	2,500
Forfeited (in shares) | shares	7,500
Exercisable at end of period (in shares) | shares	20,000
Remaining unexercised (in shares) | shares	20,000
Exercise price (in usd per share) | $ / shares	$ 54.17
Volatility	0.225
Expected life (years)	8 years
Assumed yield	4.39%
Risk-free rate	4.65%
Total value at grant date	$ 285,300
Expensed in previous years	285,300
Expensed in 2015	0
Future expense	$ 0
Directors | 4/25/2008
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 25, 2008
Total grant (in shares) | shares	30,000
Vested (in shares)	$ 30,000
Exercised (in shares) | shares	2,500
Forfeited (in shares) | shares	7,500
Exercisable at end of period (in shares) | shares	20,000
Remaining unexercised (in shares) | shares	20,000
Exercise price (in usd per share) | $ / shares	$ 50.15
Volatility	0.237
Expected life (years)	7 years
Assumed yield	4.09%
Risk-free rate	3.49%
Total value at grant date	$ 254,700
Expensed in previous years	254,700
Expensed in 2015	0
Future expense	$ 0
Directors | 4/24/2009
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 24, 2009
Total grant (in shares) | shares	32,500
Vested (in shares)	$ 32,500
Exercised (in shares) | shares	22,500
Forfeited (in shares) | shares	0
Exercisable at end of period (in shares) | shares	10,000
Remaining unexercised (in shares) | shares	10,000
Exercise price (in usd per share) | $ / shares	$ 32.68
Volatility	0.344
Expected life (years)	6 years
Assumed yield	4.54%
Risk-free rate	2.19%
Total value at grant date	$ 222,950
Expensed in previous years	222,950
Expensed in 2015	0
Future expense	$ 0
Directors | 5/7/2010
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 07, 2010
Total grant (in shares) | shares	32,500
Vested (in shares)	$ 32,500
Exercised (in shares) | shares	12,500
Forfeited (in shares) | shares	2,500
Exercisable at end of period (in shares) | shares	17,500
Remaining unexercised (in shares) | shares	17,500
Exercise price (in usd per share) | $ / shares	$ 38.76
Volatility	0.369
Expected life (years)	5 years
Assumed yield	4.23%
Risk-free rate	2.17%
Total value at grant date	$ 287,950
Expensed in previous years	287,950
Expensed in 2015	0
Future expense	$ 0
Directors | 5/13/2011
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 13, 2011
Total grant (in shares) | shares	32,500
Vested (in shares)	$ 32,500
Exercised (in shares) | shares	12,500
Forfeited (in shares) | shares	2,500
Exercisable at end of period (in shares) | shares	17,500
Remaining unexercised (in shares) | shares	17,500
Exercise price (in usd per share) | $ / shares	$ 41.82
Volatility	0.358
Expected life (years)	5 years
Assumed yield	4.16%
Risk-free rate	1.86%
Total value at grant date	$ 297,375
Expensed in previous years	297,375
Expensed in 2015	0
Future expense	$ 0
Directors | 5/4/2012
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 04, 2012
Total grant (in shares) | shares	35,000
Vested (in shares)	$ 35,000
Exercised (in shares) | shares	12,500
Forfeited (in shares) | shares	0
Exercisable at end of period (in shares) | shares	22,500
Remaining unexercised (in shares) | shares	22,500
Exercise price (in usd per share) | $ / shares	$ 39.29
Volatility	0.348
Expected life (years)	5 years
Assumed yield	4.61%
Risk-free rate	0.78%
Total value at grant date	$ 257,250
Expensed in previous years	257,250
Expensed in 2015	0
Future expense	$ 0
Directors | 5/10/2013
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 10, 2013
Total grant (in shares) | shares	35,000
Vested (in shares)	$ 35,000
Exercised (in shares) | shares	10,000
Forfeited (in shares) | shares	0
Exercisable at end of period (in shares) | shares	25,000
Remaining unexercised (in shares) | shares	25,000
Exercise price (in usd per share) | $ / shares	$ 44.42
Volatility	0.333
Expected life (years)	5 years
Assumed yield	4.53%
Risk-free rate	0.82%
Total value at grant date	$ 278,250
Expensed in previous years	278,250
Expensed in 2015	0
Future expense	$ 0
Directors | 5/9/2014
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 09, 2014
Total grant (in shares) | shares	30,000
Vested (in shares)	$ 30,000
Exercised (in shares) | shares	2,500
Forfeited (in shares) | shares	0
Exercisable at end of period (in shares) | shares	27,500
Remaining unexercised (in shares) | shares	27,500
Exercise price (in usd per share) | $ / shares	$ 47.03
Volatility	0.173
Expected life (years)	5 years
Assumed yield	4.48%
Risk-free rate	1.63%
Total value at grant date	$ 109,500
Expensed in previous years	109,500
Expensed in 2015	0
Future expense	$ 0
Directors | 5/8/2015
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 08, 2015
Total grant (in shares) | shares	35,000
Vested (in shares)	$ 35,000
Exercised (in shares) | shares	0
Forfeited (in shares) | shares	0
Exercisable at end of period (in shares) | shares	35,000
Remaining unexercised (in shares) | shares	35,000
Exercise price (in usd per share) | $ / shares	$ 51.07
Volatility	0.166
Expected life (years)	5 years
Assumed yield	4.54%
Risk-free rate	1.50%
Total value at grant date	$ 125,300
Expensed in previous years	0
Expensed in 2015	125,300
Future expense	$ 0
Directors | Subtotals
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares) | shares	322,500
Vested (in shares)	$ 322,500
Exercised (in shares) | shares	92,500
Forfeited (in shares) | shares	22,500
Exercisable at end of period (in shares) | shares	207,500
Remaining unexercised (in shares) | shares	207,500
Total value at grant date	$ 2,261,975
Expensed in previous years	2,136,675
Expensed in 2015	125,300
Future expense	$ 0
Officers
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares) | shares	1,425,000
Vested (in shares)	$ 724,375
Exercised (in shares) | shares	225,351
Forfeited (in shares) | shares	298,750
Exercisable at end of period (in shares) | shares	476,524
Remaining unexercised (in shares) | shares	900,899
Total value at grant date	$ 10,821,550
Estimated forfeitures	1,910,624
Expensed in previous years	5,432,122
Expensed in 2015	905,818
Future expense	$ 2,572,986
Officers | 4/27/2007
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 27, 2007
Total grant (in shares) | shares	135,000
Vested (in shares)	$ 67,500
Exercised (in shares) | shares	14,097
Forfeited (in shares) | shares	67,500
Exercisable at end of period (in shares) | shares	53,403
Remaining unexercised (in shares) | shares	53,403
Exercise price (in usd per share) | $ / shares	$ 54.17
Volatility	0.233
Expected life (years)	6 years 6 months
Assumed yield	4.13%
Risk-free rate	4.61%
Total value at grant date	$ 1,339,200
Estimated forfeitures	62,000
Expensed in previous years	1,277,200
Expensed in 2015	0
Future expense	$ 0
Officers | 5/13/2011
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 13, 2011
Total grant (in shares) | shares	162,500
Vested (in shares)	$ 118,750
Exercised (in shares) | shares	57,504
Forfeited (in shares) | shares	43,750
Exercisable at end of period (in shares) | shares	61,246
Remaining unexercised (in shares) | shares	61,246
Exercise price (in usd per share) | $ / shares	$ 41.82
Volatility	0.330
Expected life (years)	8 years
Assumed yield	4.81%
Risk-free rate	2.75%
Total value at grant date	$ 1,366,625
Estimated forfeitures	367,937
Expensed in previous years	909,563
Expensed in 2015	89,125
Future expense	$ 0
Officers | 5/4/2012
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 04, 2012
Total grant (in shares) | shares	242,500
Vested (in shares)	$ 81,875
Exercised (in shares) | shares	38,750
Forfeited (in shares) | shares	135,000
Exercisable at end of period (in shares) | shares	43,125
Remaining unexercised (in shares) | shares	68,750
Exercise price (in usd per share) | $ / shares	$ 39.29
Volatility	0.315
Expected life (years)	8 years
Assumed yield	5.28%
Risk-free rate	1.49%
Total value at grant date	$ 1,518,050
Estimated forfeitures	889,690
Expensed in previous years	418,899
Expensed in 2015	117,819
Future expense	$ 91,642
Officers | 5/10/2013
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 10, 2013
Total grant (in shares) | shares	202,500
Vested (in shares)	$ 91,250
Exercised (in shares) | shares	21,875
Forfeited (in shares) | shares	30,000
Exercisable at end of period (in shares) | shares	69,375
Remaining unexercised (in shares) | shares	150,625
Exercise price (in usd per share) | $ / shares	$ 44.42
Volatility	0.304
Expected life (years)	8 years
Assumed yield	5.12%
Risk-free rate	1.49%
Total value at grant date	$ 1,401,300
Estimated forfeitures	280,468
Expensed in previous years	492,937
Expensed in 2015	201,825
Future expense	$ 426,070
Officers | 5/9/2014
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 09, 2014
Total grant (in shares) | shares	170,000
Vested (in shares)	$ 42,500
Exercised (in shares) | shares	625
Forfeited (in shares) | shares	0
Exercisable at end of period (in shares) | shares	41,875
Remaining unexercised (in shares) | shares	169,375
Exercise price (in usd per share) | $ / shares	$ 47.03
Volatility	0.306
Expected life (years)	7 years
Assumed yield	4.89%
Risk-free rate	2.17%
Total value at grant date	$ 1,349,800
Estimated forfeitures	168,749
Expensed in previous years	196,848
Expensed in 2015	221,454
Future expense	$ 762,749
Officers | 5/8/2015
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 08, 2015
Total grant (in shares) | shares	190,000
Vested (in shares)	$ 0
Exercised (in shares) | shares	0
Forfeited (in shares) | shares	0
Exercisable at end of period (in shares) | shares	0
Remaining unexercised (in shares) | shares	190,000
Exercise price (in usd per share) | $ / shares	$ 51.07
Volatility	0.298
Expected life (years)	7 years
Assumed yield	4.94%
Risk-free rate	1.89%
Total value at grant date	$ 1,584,600
Estimated forfeitures	141,780
Expensed in previous years	0
Expensed in 2015	150,295
Future expense	$ 1,292,525
Officers | Subtotals
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares) | shares	1,102,500
Vested (in shares)	$ 401,875
Exercised (in shares) | shares	132,851
Forfeited (in shares) | shares	276,250
Exercisable at end of period (in shares) | shares	269,024
Remaining unexercised (in shares) | shares	693,399
Total value at grant date	$ 8,559,575
Estimated forfeitures	1,910,624
Expensed in previous years	3,295,447
Expensed in 2015	780,518
Future expense	$ 2,572,986