Summary of Significant Accounting Policies - Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of Significant Accounting Policies
Net income for basic and diluted income per common share	$ 2,286	$ 5,676	$ 2,559	$ 2,365	$ 3,731	$ 5,426	$ 5,718	$ 4,492	$ 12,886	$ 19,367	$ 14,800
Weighted-average shares for basic income per share									5,598,805	5,578,784	5,561,429
Effect of dilutive securities:
Employee and director stock options (in shares)									5,423	10,459	9,850
Adjusted weighted-average shares and assumed exercises for diluted income per share									5,604,228	5,589,243	5,571,279
Basic net income per common share (in dollars per share)	$ 0.41	$ 1.01	$ 0.46	$ 0.42	$ 0.67	$ 0.97	$ 1.03	$ 0.81	$ 2.30	$ 3.47	$ 2.66
Diluted net income per common share (in dollars per share)	$ 0.41	$ 1.01	$ 0.46	$ 0.42	$ 0.67	$ 0.97	$ 1.03	$ 0.80	$ 2.30	$ 3.47	$ 2.66
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities									24,225	7,500	15,000