Computation of Basic and Diluted Earnings Per Share (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
Earnings Per Share Disclosure [Line Items]
Net income, as reported	$ 6,412	$ 7,070	$ 12,995	$ 13,409
Weighted-average common shares outstanding - basic	7,675,819	7,677,594	7,673,927	7,668,831
Dilutive effect of stock-based compensation	11,801	9,539	12,820	10,467
Weighted-average common and potential common shares - diluted	7,687,620	7,687,133	7,687,747	7,679,298
Basic earnings per share	$ 0.84	$ 0.92	$ 1.69	$ 1.75
Diluted earnings per share	$ 0.83	$ 0.92	$ 1.69	$ 1.75
Unvested share-based payment awards
Earnings Per Share Disclosure [Line Items]
Basic earnings per share	$ 0.81	$ 0.88	$ 1.55	$ 1.64
Diluted earnings per share	$ 0.83	$ 0.92	$ 1.69	$ 1.75