Earnings per Share (Computation of Basic and Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Earnings per Share
Net Income	$ 4,178	$ 6,255	$ 6,412	$ 6,583	$ 5,839	$ 6,929	$ 7,070	$ 6,339	$ 23,428		$ 26,177		$ 24,766
Dividends and undistributed earnings allocated to participating securities									(60)	[1]	(42)	[1]	(38)	[1]
Net income available to common shareholders									$ 23,368		$ 26,135		$ 24,728
Weighted-average common shares outstanding for basic EPS									7,646,861		7,672,126		7,655,668
Dilutive effect of stock-based awards									14,412	[2]	7,769	[2]	7,830	[2]
Weighted-average common and potential common shares for diluted EPS									7,661,273		7,679,895		7,663,498
Basic EPS	$ 0.55	$ 0.82	$ 0.83	$ 0.86	$ 0.76	$ 0.9	$ 0.92	$ 0.83	$ 3.06		$ 3.41		$ 3.23
Diluted EPS	$ 0.55	$ 0.82	$ 0.83	$ 0.86	$ 0.76	$ 0.9	$ 0.92	$ 0.83	$ 3.05		$ 3.4		$ 3.23
Shares that were not considered in computation of potential common shares for purposes of diluted EPS									49,500		102,000

[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.