Summary of Significant Accounting Policies - Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Anti-dilutive securities
Net income for basic and diluted income per common share	$ 3,920	$ 6,081	$ 3,687	$ 4,066	$ 2,286	$ 5,676	$ 2,559	$ 2,365	$ 17,754	$ 12,886	$ 19,367
Weighted-average shares for basic income per share									5,567,902	5,598,805	5,578,784
Effect of dilutive securities:
Employee and director stock options (in shares)									4,071	5,423	10,459
Adjusted weighted-average shares and assumed exercises for diluted income per share									5,571,973	5,604,228	5,589,243
Basic net income per common share (in dollars per share)	$ 0.70	$ 1.09	$ 0.66	$ 0.73	$ 0.41	$ 1.01	$ 0.46	$ 0.42	$ 3.19	$ 2.30	$ 3.47
Diluted net income per common share (in dollars per share)	$ 0.70	$ 1.09	$ 0.66	$ 0.73	$ 0.41	$ 1.01	$ 0.46	$ 0.42	$ 3.19	$ 2.30	$ 3.47
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities									22,425	24,225	7,500