Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, except 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 per share of common stock
Net income for basic and diluted income per common share	$ 3,792	$ 3,947	$ 4,060	$ 4,624	$ 4,515	$ 7,062	$ 5,795	$ 4,813	$ 16,423	$ 22,186	$ 18,040
Weighted-average shares for basic income per common share									5,705,475	6,343,992	6,400,958
Effect of dilutive securities:
Employee and director stock options (in shares)									10,286	18,449	16,859
Adjusted weighted-average shares and assumed exercises for diluted income per common share									5,715,761	6,362,441	6,417,817
Basic net income per common share	$ 0.68	$ 0.71	$ 0.73	$ 0.75	$ 0.73	$ 1.12	$ 0.90	$ 0.75	$ 2.88	$ 3.50	$ 2.82
Diluted net income per common share	$ 0.68	$ 0.71	$ 0.73	$ 0.75	$ 0.72	$ 1.11	$ 0.90	$ 0.75	$ 2.87	$ 3.49	$ 2.81
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities									17,400	7,500	9,500