Stock Option Plans - Stock Options Issued (Details)	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares shares
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Exercisable at end of period (in shares)	421,524
Exercise price (in usd per share) | $ / shares	$ 42.00
Weighted average term of remaining future expense (in years)	2 years 5 months 21 days
Directors | 5/1/2006
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)	25,000
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	$ 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 2016 | $	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)	5,000
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	17,500
Remaining unexercised (in shares)	17,500
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 2016 | $	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)	7,500
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	15,000
Remaining unexercised (in shares)	15,000
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 2016 | $	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)	22,500
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	$ 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 2016 | $	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)	15,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	15,000
Remaining unexercised (in shares)	15,000
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 2016 | $	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)	15,000
Forfeited (in shares)	2,500
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	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 2016 | $	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)	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	$ 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 2016 | $	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)	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	$ 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 2016 | $	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)	7,500
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	$ 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 2016 | $	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	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 | $	125,300
Expensed in 2016 | $	0
Future expense | $	$ 0
Directors | Subtotals
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)	130,000
Forfeited (in shares)	22,500
Exercisable at end of period (in shares)	170,000
Remaining unexercised (in shares)	170,000
Total value at grant date | $	$ 2,261,975
Expensed in previous years | $	2,261,975
Expensed in 2016 | $	0
Future expense | $	$ 0
Officers
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,425,000
Vested (in shares)	724,375
Exercised (in shares)	280,351
Forfeited (in shares)	298,750
Exercisable at end of period (in shares)	421,524
Remaining unexercised (in shares)	845,899
Total value at grant date | $	$ 10,821,550
Estimated forfeitures | $	1,910,624
Expensed in previous years | $	6,608,483
Expensed in 2016 | $	270,543
Future expense | $	$ 2,031,900
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)	14,097
Forfeited (in shares)	67,500
Exercisable at end of period (in shares)	53,403
Remaining unexercised (in 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 2016 | $	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)	61,254
Forfeited (in shares)	43,750
Exercisable at end of period (in shares)	57,496
Remaining unexercised (in shares)	57,496
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 | $	998,688
Expensed in 2016 | $	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)	81,875
Exercised (in shares)	43,125
Forfeited (in shares)	135,000
Exercisable at end of period (in shares)	38,750
Remaining unexercised (in shares)	64,375
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 | $	575,994
Expensed in 2016 | $	39,273
Future expense | $	$ 13,093
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)	91,250
Exercised (in shares)	26,875
Forfeited (in shares)	30,000
Exercisable at end of period (in shares)	64,375
Remaining unexercised (in shares)	145,625
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 | $	762,034
Expensed in 2016 | $	67,275
Future expense | $	$ 291,523
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)	42,500
Exercised (in shares)	5,000
Forfeited (in shares)	0
Exercisable at end of period (in shares)	37,500
Remaining unexercised (in shares)	165,000
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 | $	492,120
Expensed in 2016 | $	73,818
Future expense | $	$ 615,113
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)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Exercisable at end of period (in shares)	0
Remaining unexercised (in 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 | $	240,472
Expensed in 2016 | $	90,177
Future expense | $	$ 1,112,171
Officers | Subtotals
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,102,500
Vested (in shares)	401,875
Exercised (in shares)	150,351
Forfeited (in shares)	276,250
Exercisable at end of period (in shares)	251,524
Remaining unexercised (in shares)	675,899
Total value at grant date | $	$ 8,559,575
Estimated forfeitures | $	1,910,624
Expensed in previous years | $	4,346,508
Expensed in 2016 | $	270,543
Future expense | $	$ 2,031,900