Stock Options Issued (Detail) (USD $)	6 Months Ended
	Jun. 30, 2013
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Exercisable at June 30, 2013	453,000
Exercise price	$ 34.19
Director
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant	317,500
Vested	$ 300,000
Exercised	65,000
Forfeited	17,500
Exercisable at June 30, 2013	235,000
Remaining unexercised	235,000
Total value at grant date	2,136,709
Expensed in previous years	1,873,763
Expensed in 2013	262,946
Director | Period 1
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 26, 2004
Total grant	30,000
Vested	30,000
Exercised	22,500
Exercisable at June 30, 2013	7,500
Remaining unexercised	7,500
Exercise price	$ 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
Director | Period 2
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 06, 2005
Total grant	30,000
Vested	30,000
Exercised	20,000
Exercisable at June 30, 2013	10,000
Remaining unexercised	10,000
Exercise price	$ 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
Director | Period 3
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 01, 2006
Total grant	30,000
Vested	27,500
Exercised	2,500
Forfeited	2,500
Exercisable at June 30, 2013	25,000
Remaining unexercised	25,000
Exercise price	$ 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
Director | Period 4
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 27, 2007
Total grant	30,000
Vested	25,000
Forfeited	5,000
Exercisable at June 30, 2013	25,000
Remaining unexercised	25,000
Exercise price	$ 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
Director | Period 5
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 25, 2008
Total grant	30,000
Vested	25,000
Forfeited	5,000
Exercisable at June 30, 2013	25,000
Remaining unexercised	25,000
Exercise price	$ 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
Director | Period 6
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 24, 2009
Total grant	32,500
Vested	32,500
Exercised	12,500
Exercisable at June 30, 2013	20,000
Remaining unexercised	20,000
Exercise price	$ 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
Director | Period 7
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 07, 2010
Total grant	32,500
Vested	30,000
Exercised	2,500
Forfeited	2,500
Exercisable at June 30, 2013	27,500
Remaining unexercised	27,500
Exercise price	$ 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
Director | Period 8
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 13, 2011
Total grant	32,500
Vested	30,000
Exercised	2,500
Forfeited	2,500
Exercisable at June 30, 2013	27,500
Remaining unexercised	27,500
Exercise price	$ 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
Director | Period 9
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 04, 2012
Total grant	35,000
Vested	35,000
Exercised	2,500
Exercisable at June 30, 2013	32,500
Remaining unexercised	32,500
Exercise price	$ 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
Director | Period 10
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 10, 2013
Total grant	35,000
Vested	35,000
Exercisable at June 30, 2013	35,000
Remaining unexercised	35,000
Exercise price	$ 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 2013	262,946
Officer
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant	1,315,000
Vested	696,875
Exercised	243,875
Forfeited	277,500
Exercisable at June 30, 2013	453,000
Remaining unexercised	793,625
Total value at grant date	8,075,838
Forfeited options	1,119,125
Expensed in previous years	4,347,223
Expensed in 2013	511,423
Future expense	2,098,067
Officer | Period 1
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 26, 2004
Total grant	122,500
Vested	115,000
Exercised	91,250
Forfeited	7,500
Exercisable at June 30, 2013	23,750
Remaining unexercised	23,750
Exercise price	$ 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
Officer | Period 2
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 06, 2005
Total grant	132,500
Vested	118,750
Exercised	69,500
Forfeited	13,750
Exercisable at June 30, 2013	49,250
Remaining unexercised	49,250
Exercise price	$ 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
Officer | Period 3
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	Apr. 27, 2007
Total grant	135,000
Vested	67,500
Forfeited	67,500
Exercisable at June 30, 2013	67,500
Remaining unexercised	67,500
Exercise price	$ 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
Expensed in previous years	1,258,848
Weighted average term of remaining future expense	3 years 2 months 12 days
Officer | Period 4
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 13, 2011
Total grant	162,500
Vested	67,500
Exercised	16,250
Forfeited	41,250
Exercisable at June 30, 2013	51,250
Remaining unexercised	105,000
Exercise price	$ 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	456,738
Expensed in 2013	117,678
Future expense	451,078
Officer | Period 5
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 04, 2012
Total grant	242,500
Vested	28,125
Exercised	1,875
Forfeited	130,000
Exercisable at June 30, 2013	26,250
Remaining unexercised	110,625
Exercise price	$ 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	104,724
Expensed in 2013	78,543
Future expense	445,081
Officer | Period 6
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Grant date	May 10, 2013
Total grant	202,500
Remaining unexercised	202,500
Exercise price	$ 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
Expensed in 2013	52,257
Future expense	1,201,907
Officer | Subtotal
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Total grant	997,500
Vested	396,875
Exercised	178,875
Forfeited	260,000
Exercisable at June 30, 2013	218,000
Remaining unexercised	558,625
Total value at grant date	5,939,129
Forfeited options	1,119,125
Expensed in previous years	2,473,460
Expensed in 2013	248,477
Future expense	$ 2,098,067