Computation of Basic and Diluted Earnings Per Share Under Two-Class Method (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator for basic and diluted earnings per share:
Net income	$ 31,917	$ 29,944	$ 62,208	$ 60,108
Effect of reallocating undistributed earnings of participating securities	(171)	(174)	(336)	(350)
Net income available under the two-class method	$ 31,746	$ 29,770	$ 61,872	$ 59,758
Denominator:
Denominator for basic earnings per share- weighted-average shares	22,961,130	23,027,104	22,967,558	23,020,851
Effect of dilutive securities:
Dilutive potential common shares	1,220	1,346	1,250	1,324
Denominator for diluted earnings per share- adjusted weighted-average shares	22,962,350	23,028,450	22,968,808	23,022,175
Basic earnings per share	$ 1.38	$ 1.29	$ 2.69	$ 2.60
Diluted earnings per share	$ 1.38	$ 1.29	$ 2.69	$ 2.60
Employee Stock Options
Effect of dilutive securities:
Employee stock options	1,220	1,346	1,250	1,324