Quarterly Condensed Statements of Income-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, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement [Abstract]
Total interest income	$ 19,605	$ 19,505	$ 19,098	$ 18,756	$ 18,871	$ 18,918	$ 18,369	$ 17,632	$ 76,964	$ 73,790	$ 69,848
Net interest income after provision for loan losses	13,485	14,129	13,642	13,192	12,078	11,912	12,011	11,748	54,448	47,749	41,654
Other income	8,820	9,570	7,729	7,383	7,091	7,140	6,358	6,457	2,266	1,132	1,808
Other expenses	16,651	16,987	15,227	15,057	14,243	13,923	13,969	13,949	16,434	15,276	19,638
Income before income taxes	5,654	6,712	6,144	5,518	4,926	5,129	4,400	4,256	24,028	18,711	11,059
Net income	3,888	4,533	4,181	3,780	3,411	3,513	3,083	2,969	16,382	12,976	8,110
Net income available to common shareholders	$ 3,888	$ 4,533	$ 4,016	$ 3,634	$ 3,265	$ 3,055	$ 2,793	$ 2,680	$ 16,071	$ 11,793	$ 6,961
Earnings per common share-assuming dilution (in dollars per share)	$ 1.17	$ 1.36	$ 1.22	$ 1.11	$ 1.02	$ 0.96	$ 0.88	$ 0.85	$ 4.86	$ 3.72	$ 2.24
Dividends declared per common share (in dollars per share)	$ 0.29	$ 0.27	$ 0.26	$ 0.26	$ 0.26	$ 0.25	$ 0.25	$ 0.25	$ 1.08	$ 1.01	$ 1.00