Stock Option Plans - Stock Options Issued (Detail) (USD $)	3 Months Ended
Mar. 31, 2015
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Exercisable at end of period (in shares)	314,024
Exercise price (in usd per share)	$ 43.12
Weighted average term of remaining future expense	2 years 6 months 4 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)	27,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	2,500
Remaining unexercised (in shares)	2,500
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 2015	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)	12,500
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)	$ 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)	30,000
Vested (in shares)	30,000
Exercised (in shares)	2,500
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	20,000
Remaining unexercised (in shares)	20,000
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 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)	30,000
Vested (in shares)	30,000
Exercised (in shares)	2,500
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	20,000
Remaining unexercised (in shares)	20,000
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 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)	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)	$ 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)	32,500
Vested (in shares)	32,500
Exercised (in shares)	12,500
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)	$ 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)	32,500
Vested (in shares)	32,500
Exercised (in shares)	12,500
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)	$ 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)	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)	$ 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)	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)	$ 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)	30,000
Vested (in shares)	30,000
Exercised (in shares)	2,500
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)	$ 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 \| 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)	117,500
Forfeited (in shares)	22,500
Exercisable at end of period (in shares)	177,500
Remaining unexercised (in shares)	177,500
Total value at grant date	2,207,775
Expensed in previous years	2,207,775
Expensed in 2015	0
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)	368,476
Forfeited (in shares)	312,500
Exercisable at end of period (in shares)	314,024
Remaining unexercised (in shares)	681,524
Total value at grant date	9,596,150
Estimated forfeitures	1,823,557
Expensed in previous years	5,881,522
Expensed in 2015	232,500
Future expense	1,658,571
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)	118,750
Forfeited (in shares)	13,750
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	0
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 2015	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)	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)	$ 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)	162,500
Vested (in shares)	105,625
Exercised (in shares)	57,504
Forfeited (in shares)	43,750
Exercisable at end of period (in shares)	36,871
Remaining unexercised (in shares)	61,246
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	909,563
Expensed in 2015	52,134
Future expense	17,378
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)	38,750
Forfeited (in shares)	135,000
Exercisable at end of period (in shares)	17,500
Remaining unexercised (in shares)	68,750
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	418,899
Expensed in 2015	39,273
Future expense	170,188
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)	21,875
Forfeited (in shares)	30,000
Exercisable at end of period (in shares)	28,750
Remaining unexercised (in shares)	150,625
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	492,937
Expensed in 2015	67,275
Future expense	560,620
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	196,848
Expensed in 2015	73,818
Future expense	910,385
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)	250,976
Forfeited (in shares)	290,000
Exercisable at end of period (in shares)	136,524
Remaining unexercised (in shares)	504,024
Total value at grant date	7,388,375
Estimated forfeitures	1,823,557
Expensed in previous years	3,673,747
Expensed in 2015	232,500
Future expense	$ 1,658,571

Stock Option Plans - Stock Options Issued (Detail) (USD $)

3 Months Ended

Mar. 31, 2015

Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]

Exercisable at end of period (in shares)

314,024

Exercise price (in usd per share)

$ 43.12

Weighted average term of remaining future expense

2 years 6 months 4 days

Directors | 5/6/2005

Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]