Stock Option Plans - Stock Options Issued (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Exercisable at end of period (in shares)	417,059
Exercise price (in usd per share)	$ 37.30
Weighted average term of remaining future expense	2 years 10 months 8 days
Directors | 5/6/2005
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 06, 2005
Total grant (in shares)	30,000
Vested (in shares)	$ 30,000
Exercised (in shares)	22,500
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)	$ 33.22
Volatility	0.198
Expected life (years)	10 years
Assumed yield	6.91%
Risk-free rate	4.28%
Total value at grant date	71,100
Expensed in previous years	71,100
Expensed in 2014	0
Future expense	0
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)	30,000
Vested (in shares)	30,000
Exercised (in shares)	10,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	17,500
Remaining unexercised (in shares)	17,500
Exercise price (in usd per share)	$ 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 2014	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)	30,000
Vested (in shares)	30,000
Exercised (in shares)	0
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	22,500
Remaining unexercised (in shares)	22,500
Exercise price (in usd per share)	$ 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 2014	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)	30,000
Vested (in shares)	30,000
Exercised (in shares)	0
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	22,500
Remaining unexercised (in shares)	22,500
Exercise price (in usd per share)	$ 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 2014	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)	32,500
Vested (in shares)	32,500
Exercised (in shares)	20,000
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)	$ 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 2014	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)	32,500
Vested (in shares)	32,500
Exercised (in shares)	10,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	20,000
Remaining unexercised (in shares)	20,000
Exercise price (in usd per share)	$ 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 2014	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)	32,500
Vested (in shares)	32,500
Exercised (in shares)	10,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	20,000
Remaining unexercised (in shares)	20,000
Exercise price (in usd per share)	$ 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 2014	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)	35,000
Vested (in shares)	35,000
Exercised (in shares)	10,000
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)	$ 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 2014	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)	35,000
Vested (in shares)	35,000
Exercised (in shares)	7,500
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)	$ 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 2014	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)	30,000
Vested (in shares)	30,000
Exercised (in shares)	0
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)	$ 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	0
Expensed in 2014	109,500
Future expense	0
Directors | Subtotals
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	317,500
Vested (in shares)	317,500
Exercised (in shares)	90,000
Forfeited (in shares)	22,500
Exercisable at end of period (in shares)	205,000
Remaining unexercised (in shares)	205,000
Total value at grant date	2,207,775
Expensed in previous years	2,098,275
Expensed in 2014	109,500
Future expense	0
Officers
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,362,500
Vested (in shares)	716,250
Exercised (in shares)	265,441
Forfeited (in shares)	312,500
Exercisable at end of period (in shares)	417,059
Remaining unexercised (in shares)	784,559
Total value at grant date	9,596,150
Estimated forfeitures	1,823,557
Expensed in previous years	4,916,697
Expensed in 2014	732,325
Future expense	2,123,571
Officers | 5/6/2005
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 06, 2005
Total grant (in shares)	132,500
Vested (in shares)	118,750
Exercised (in shares)	102,316
Forfeited (in shares)	13,750
Exercisable at end of period (in shares)	16,434
Remaining unexercised (in shares)	16,434
Exercise price (in usd per share)	$ 33.22
Volatility	0.207
Expected life (years)	8 years
Assumed yield	6.37%
Risk-free rate	4.15%
Total value at grant date	413,400
Estimated forfeitures	35,100
Expensed in previous years	378,300
Expensed in 2014	0
Future expense	0
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)	135,000
Vested (in shares)	67,500
Exercised (in shares)	0
Forfeited (in shares)	67,500
Exercisable at end of period (in shares)	67,500
Remaining unexercised (in shares)	67,500
Exercise price (in usd per share)	$ 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 2014	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)	162,500
Vested (in shares)	105,625
Exercised (in shares)	38,125
Forfeited (in shares)	43,750
Exercisable at end of period (in shares)	56,250
Remaining unexercised (in shares)	80,625
Exercise price (in usd per share)	$ 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	387,550
Expensed in previous years	692,088
Expensed in 2014	165,341
Future expense	121,646
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)	242,500
Vested (in shares)	56,250
Exercised (in shares)	20,000
Forfeited (in shares)	135,000
Exercisable at end of period (in shares)	36,250
Remaining unexercised (in shares)	87,500
Exercise price (in usd per share)	$ 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	261,810
Expensed in 2014	117,819
Future expense	248,731
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)	202,500
Vested (in shares)	50,625
Exercised (in shares)	15,000
Forfeited (in shares)	30,000
Exercisable at end of period (in shares)	35,625
Remaining unexercised (in shares)	157,500
Exercise price (in usd per share)	$ 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	209,024
Expensed in 2014	216,635
Future expense	695,173
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)	170,000
Vested (in shares)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	170,000
Exercise price (in usd per share)	$ 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	0
Expensed in 2014	123,030
Future expense	1,058,021
Officers | Subtotals
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,045,000
Vested (in shares)	398,750
Exercised (in shares)	175,441
Forfeited (in shares)	290,000
Exercisable at end of period (in shares)	212,059
Remaining unexercised (in shares)	579,559
Total value at grant date	7,388,375
Estimated forfeitures	1,823,557
Expensed in previous years	2,818,422
Expensed in 2014	622,825
Future expense	$ 2,123,571