Quarterly Operating Results (unaudited) (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, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Operating Results (unaudited)
Interest income	$ 22,034	$ 21,692	$ 21,363	$ 21,812	$ 21,569	$ 21,616	$ 21,566	$ 21,288	$ 21,723	$ 22,018	$ 21,448	$ 20,957	$ 86,901	$ 86,039	$ 86,146
Interest expense	3,724	2,897	3,068	3,262	3,553	3,826	3,955	3,973	4,399	4,804	4,901	5,163	12,951	15,307	19,267
Net interest income	18,310	18,795	18,295	18,550	18,016	17,790	17,611	17,315	17,324	17,214	16,547	15,794	73,950	70,732	66,879
Provision for loan losses	2,475	2,475	2,475	4,075	3,100	4,100	2,600	2,800	3,695	2,695	2,384	2,695	11,500	12,600	11,469
Net interest income after provision for loan losses	15,835	16,320	15,820	14,475	14,916	13,690	15,011	14,515	13,629	14,519	14,163	13,099	62,450	58,132	55,410
Non-interest income	10,127	9,795	9,290	9,245	9,229	7,858	8,152	8,005	9,578	8,262	7,923	7,976	38,457	33,244	33,739
Non-interest expenses	17,183	17,045	16,508	14,736	16,727	13,302	14,725	14,827	15,083	13,909	14,381	13,758	65,472	59,581	57,131
Income before income taxes	8,779	9,070	8,602	8,984	7,418	8,246	8,438	7,693	8,124	8,872	7,705	7,317	35,435	31,795	32,018
Income tax expense	2,265	2,388	2,499	2,482	1,076	2,472	2,441	2,202	2,073	2,507	2,149	2,336	9,634	8,191	9,065
Net income	$ 6,514	$ 6,682	$ 6,103	$ 6,502	$ 6,342	$ 5,774	$ 5,997	$ 5,491	$ 6,051	$ 6,365	$ 5,556	$ 4,981	$ 25,801	$ 23,604	$ 22,953
Basic earnings per share (in dollars per share)	$ 0.47	$ 0.48	$ 0.44	$ 0.47	$ 0.46	$ 0.42	$ 0.43	$ 0.40	$ 0.44	$ 0.46	$ 0.41	$ 0.37	$ 1.86	$ 1.71	$ 1.68
Diluted earnings per share (in dollars per share)	$ 0.47	$ 0.48	$ 0.44	$ 0.47	$ 0.46	$ 0.42	$ 0.43	$ 0.40	$ 0.44	$ 0.46	$ 0.40	$ 0.36	$ 1.85	$ 1.71	$ 1.67