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, 2014	Mar. 31, 2013
Numerator for basic and diluted earnings per share:
Net income	$ 18,427	$ 17,577
Effect of reallocating undistributed earnings of participating securities	(136)	(117)
Net income available under the two-class method	$ 18,291	$ 17,460
Denominator:
Denominator for basic earnings per share-weighted-average shares	22,948,109	22,990,048
Effect of dilutive potential common shares
Denominator for diluted earnings per share-adjusted weighted-average shares	22,948,109	22,990,048
Basic earnings per share	$ 0.80	$ 0.76
Diluted earnings per share	$ 0.80	$ 0.76