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
	Mar. 31, 2012	Mar. 31, 2011
Numerator for basic and diluted earnings per share:
Net income	$ 30,291	$ 30,164
Effect of reallocating undistributed earnings of participating securities	(165)	(176)
Net income available under the two-class method	$ 30,126	$ 29,988
Denominator:
Denominator for basic earnings per share-weighted-average shares	22,973,986	23,014,530
Effect of dilutive securities:
Dilutive potential common shares	1,280	1,302
Denominator for diluted earnings per share-adjusted weighted-average shares	22,975,266	23,015,832
Basic earnings per share	$ 1.31	$ 1.30
Diluted earnings per share	$ 1.31	$ 1.30
Employee Stock Options
Effect of dilutive securities:
Share based payment arrangements (See Note 5)	1,280	1,302