Summary of Significant Accounting Policies - Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Anti-dilutive securities
Net income for basic and diluted income per common share	$ 4,231	$ 4,554	$ 6,638	$ 4,262	$ 11,583	$ 5,667	$ 5,278	$ 4,620	$ 19,685	$ 27,148	$ 17,754
Weighted-average shares for basic income per common share (in shares)									5,595,384	5,577,312	5,567,902
Effect of dilutive securities:
Employee and director stock options (in shares)									6,993	11,184	4,071
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)									5,602,377	5,588,496	5,571,973
Basic net income per common share (in dollars per share)	$ 0.76	$ 0.81	$ 1.19	$ 0.76	$ 2.08	$ 1.02	$ 0.95	$ 0.83	$ 3.52	$ 4.87	$ 3.19
Diluted net income per common share (in dollars per share)	$ 0.75	$ 0.81	$ 1.18	$ 0.76	$ 2.07	$ 1.01	$ 0.94	$ 0.83	3.51	4.86	3.19
Cash dividends per share of common stock (in dollars per share)									$ 0.54	$ 0.54	$ 0.50
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities									9,900	0	22,425