Stock Option Plans - Stock Options Issued (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Exercisable at end of period (in shares)	381,400
Exercise price (in usd per share)	$ 29.39
Weighted average term of remaining future expense	2 years 6 months
Director | 4/26/2004
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 26, 2004
Total grant (in shares)	30,000
Vested (in shares)	$ 30,000
Exercised (in shares)	30,000
Forfeited (in shares)	0
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	0
Exercise price (in usd per share)	$ 25.78
Volatility	0.183
Expected life (years)	5 years
Assumed yield	5.75%
Risk-free rate	3.57%
Total value at grant date	66,600
Expensed in previous years	66,600
Expensed in 2013	0
Future expense	0
Director | 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
Director | 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)	5,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	22,500
Remaining unexercised (in shares)	22,500
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
Director | 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
Director | 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
Director | 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)	17,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	15,000
Remaining unexercised (in shares)	15,000
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 2013	0
Future expense	0
Director | 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)	5,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	25,000
Remaining unexercised (in shares)	25,000
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
Director | 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)	5,000
Forfeited (in shares)	2,500
Exercisable at end of period (in shares)	25,000
Remaining unexercised (in shares)	25,000
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
Director | 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)	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)	$ 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
Director | 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)	2,500
Forfeited (in shares)	0
Exercisable at end of period (in shares)	32,500
Remaining unexercised (in shares)	32,500
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
Director | 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)	92,500
Forfeited (in shares)	22,500
Exercisable at end of period (in shares)	202,500
Remaining unexercised (in shares)	202,500
Total value at grant date	2,136,709
Expensed in previous years	2,136,709
Expensed in 2013	0
Future expense	0
Officer
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	1,315,000
Vested (in shares)	714,375
Exercised (in shares)	310,475
Forfeited (in shares)	282,500
Exercisable at end of period (in shares)	381,400
Remaining unexercised (in shares)	722,025
Total value at grant date	8,075,838
Forfeited options	1,119,125
Expensed in previous years	5,211,629
Expensed in 2013	176,493
Future expense	1,568,591
Officer | 4/26/2004
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 26, 2004
Total grant (in shares)	122,500
Vested (in shares)	115,000
Exercised (in shares)	115,000
Forfeited (in shares)	7,500
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	0
Exercise price (in usd per share)	$ 25.78
Volatility	0.183
Expected life (years)	7 years
Assumed yield	5.75%
Risk-free rate	4.05%
Total value at grant date	292,775
Forfeited options	17,925
Expensed in previous years	274,850
Expensed in 2013	0
Future expense	0
Officer | 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
Forfeited options	35,100
Expensed in previous years	378,300
Expensed in 2013	0
Future expense	0
Officer | 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	1,258,848
Forfeited options	0
Expensed in previous years	1,258,848
Expensed in 2013	0
Future expense	0
Officer | 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)	67,500
Exercised (in shares)	16,250
Forfeited (in shares)	41,250
Exercisable at end of period (in shares)	51,250
Remaining unexercised (in shares)	105,000
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	1,277,794
Forfeited options	252,300
Expensed in previous years	692,088
Expensed in 2013	58,838
Future expense	274,568
Officer | 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)	28,125
Exercised (in shares)	1,875
Forfeited (in shares)	130,000
Exercisable at end of period (in shares)	26,250
Remaining unexercised (in shares)	110,625
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	1,442,148
Forfeited options	813,800
Expensed in previous years	261,807
Expensed in 2013	39,271
Future expense	327,270
Officer | 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)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Exercisable at end of period (in shares)	0
Remaining unexercised (in shares)	202,500
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	1,254,164
Forfeited options	0
Expensed in previous years	209,027
Expensed in 2013	78,384
Future expense	966,753
Officer | Subtotals
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant (in shares)	997,500
Vested (in shares)	396,875
Exercised (in shares)	217,975
Forfeited (in shares)	260,000
Exercisable at end of period (in shares)	178,900
Remaining unexercised (in shares)	519,525
Total value at grant date	5,939,129
Forfeited options	1,119,125
Expensed in previous years	3,074,920
Expensed in 2013	176,493
Future expense	$ 1,568,591