Quarterly Financial Data (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
Quarterly results of operations
Total interest and dividend income	$ 16,965	$ 17,133	$ 17,207	$ 16,824	$ 16,808	$ 16,793	$ 16,417	$ 15,694	$ 68,129	$ 65,712	$ 61,147
Net interest income	13,588	13,670	13,274	12,677	12,544	12,378	11,912	11,299	53,209	48,133	42,091
Provision for loan losses	2,343	1,188	1,746	1,104	2,264	904	992	777	6,381	4,937	5,158
Non-interest income	2,803	3,079	3,264	3,518	3,454	2,462	2,616	2,187	12,664	10,719	9,650
Non-interest expense	8,890	9,208	8,833	9,014	8,540	8,310	8,141	8,291	35,945	33,282	31,798
Net income	$ 3,890	$ 4,563	$ 4,336	$ 4,347	$ 3,869	$ 4,067	$ 3,891	$ 3,277	$ 17,136	$ 15,104	$ 11,316
Basic earnings per common share	$ 0.31	$ 0.37	$ 0.35	$ 0.35	$ 0.31	$ 0.33	$ 0.32	$ 0.27	$ 1.38	$ 1.23	$ 1.06
Diluted earnings per common share	$ 0.31	$ 0.37	$ 0.35	$ 0.35	$ 0.31	$ 0.33	$ 0.32	$ 0.27	$ 1.38	$ 1.23	$ 1.06