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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator for basic and diluted earnings per share:
Net income	$ 23,017	$ 16,307	$ 41,444	$ 33,883
Effect of reallocating undistributed earnings of participating securities	(154)	(107)	(290)	(224)
Net income available under the two-class method	$ 22,863	$ 16,200	$ 41,154	$ 33,659
Denominator:
Denominator for basic earnings per share- weighted-average shares	22,947,957	22,953,453	22,948,032	22,971,649
Effect of dilutive potential common shares
Denominator for diluted earnings per share- adjusted weighted-average shares	22,947,957	22,953,453	22,948,032	22,971,649
Basic earnings per share	$ 1.00	$ 0.71	$ 1.79	$ 1.47
Diluted earnings per share	$ 1.00	$ 0.71	$ 1.79	$ 1.47