Loans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011 item	Dec. 31, 2010
Composition of loans by primary loan classification
Loans	$ 1,584,594,000	$ 1,544,845,000	$ 1,508,425,000
Net deferred loan fees	47,000	32,000
Loans under participation agreements with other banks	7,658,000	30,324,000
Loans to directors and their associates, including loans to companies for which directors are principal owners, and executive officers
Balance at the beginning of the period	622,000	642,000
New loans and advances on lines of credit	6,691,000	11,924,000
Repayments on loans and lines of credit	1,214,000	11,944,000
Balance at the end of the period	6,099,000	622,000
Number of directors owning entity to which higher amounts of advances and repayments are attributable		1
Commercial and industrial
Composition of loans by primary loan classification
Loans	426,930,000	393,729,000	343,956,000
Construction and development
Composition of loans by primary loan classification
Loans	131,253,000	147,637,000	159,482,000
Real estate mortgage
Composition of loans by primary loan classification
Loans	989,631,000	966,665,000	968,440,000
Consumer
Composition of loans by primary loan classification
Loans	$ 36,780,000	$ 36,814,000	$ 36,547,000