Computation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Earnings Per Share Disclosure [Line Items]
Net income	$ 6,255		$ 6,929		$ 19,250		$ 20,338
Dividends and undistributed earnings allocated to participating securities	(18)	[1]	(12)	[1]	(48)	[1]	(33)	[1]
Net income available to common shareholders	$ 6,237		$ 6,917		$ 19,202		$ 20,305
Weighted-average shares outstanding including participating securities	7,641,866		7,691,378		7,675,088		7,684,918
Less: average participating securities	(22,455)		(13,406)		(19,469)		(13,007)
Weighted-average common shares outstanding for basic EPS	7,619,411		7,677,972		7,655,619		7,671,911
Dilutive effect of stock-based awards	20,023	[2]	5,598	[2]	14,144	[2]	8,490	[2]
Weighted-average common and potential common shares for diluted EPS	7,639,434		7,683,570		7,669,763		7,680,401
Basic EPS	$ 0.82		$ 0.90		$ 2.51		$ 2.65
Diluted EPS	$ 0.82		$ 0.90		$ 2.50		$ 2.65

[1] Represents dividends paid and undistributed earnings allocated to nonvested restricted stock awards.

[2] Represents the effect of the assumed exercise of stock options, vesting of restricted shares, and vesting of restricted stock units, based on the treasury stock method.