Earnings Per Share (Details) (USD $) In Thousands, except Per 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
Basic and diluted earnings per share
Numerator for basic and diluted earnings per share-income available to common shareholders	$ 5,076	$ 5,770	$ 4,763	$ 5,263	$ 5,260	$ 5,244	$ 4,516	$ 3,862	$ 20,872	$ 18,882	$ 15,756
Denominator for basic earnings per share-weighted-average shares outstanding									16,761	16,743	16,598
Effect of dilutive securities-employee stock options and awards									55
Denominator for diluted earnings per share-adjusted weighted-average shares outstanding									16,816	16,743	16,598
Basic earnings per share	$ 0.30	$ 0.34	$ 0.28	$ 0.31	$ 0.32	$ 0.31	$ 0.27	$ 0.23	$ 1.25	$ 1.13	$ 0.95
Diluted earnings per share	$ 0.30	$ 0.34	$ 0.28	$ 0.31	$ 0.32	$ 0.31	$ 0.27	$ 0.23	$ 1.24	$ 1.13	$ 0.95
Average anti-dilutive options and awards excluded from computation of diluted earnings per share									581	504	403