Stock Option Plans - Stock Options Issued (Detail) (USD $)	6 Months Ended
	Jun. 30, 2014
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Exercisable at end of period (in shares)	448,940
Exercise price (in usd per share)	$ 37.23
Weighted average term of remaining future expense	3 years 1 month 6 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 2013	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 2013	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 2013	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 2013	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 2013	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 2013	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 2013	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	244,388
Expensed in previous years	244,388
Expensed in 2013	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	262,946
Expensed in previous years	262,946
Expensed in 2013	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)	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	101,726
Expensed in previous years	0
Expensed in 2013	101,726
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	2,171,835
Expensed in previous years	2,070,109
Expensed in 2013	101,726
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)	233,560
Forfeited (in shares)	312,500
Exercisable at end of period (in shares)	448,940
Remaining unexercised (in shares)	816,440
Total value at grant date	9,607,810
Estimated forfeitures	1,847,858
Expensed in previous years	4,870,181
Expensed in 2013	493,784
Future expense	2,395,987
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)	84,850
Forfeited (in shares)	13,750
Exercisable at end of period (in shares)	33,900
Remaining unexercised (in shares)	33,900
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 2013	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	80,350
Expensed in previous years	1,258,850
Expensed in 2013	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)	30,955
Forfeited (in shares)	43,750
Exercisable at end of period (in shares)	63,420
Remaining unexercised (in shares)	87,795
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 2013	113,205
Future expense	173,782
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)	18,902
Forfeited (in shares)	135,000
Exercisable at end of period (in shares)	37,348
Remaining unexercised (in shares)	88,598
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	261,807
Expensed in 2013	78,546
Future expense	288,007
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)	8,853
Forfeited (in shares)	30,000
Exercisable at end of period (in shares)	41,772
Remaining unexercised (in shares)	163,647
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	209,027
Expensed in 2013	149,360
Future expense	762,445
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	1,397,400
Estimated forfeitures	174,700
Expensed in previous years	0
Expensed in 2013	50,947
Future expense	1,171,753
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)	143,560
Forfeited (in shares)	290,000
Exercisable at end of period (in shares)	243,940
Remaining unexercised (in shares)	611,440
Total value at grant date	7,435,975
Estimated forfeitures	1,847,858
Expensed in previous years	2,800,072
Expensed in 2013	392,058
Future expense	$ 2,395,987