Summary of Significant Accounting Policies - Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Anti-dilutive securities
Net income for basic and diluted income per common share	$ 7,254	$ 9,324	$ 6,101	$ 5,544	$ 4,993	$ 9,902	$ 6,033	$ 5,128	$ 28,223	$ 26,056	$ 19,685
Weighted-average shares for basic income per common share (in shares)									5,629,425	5,612,048	5,595,384
Effect of dilutive securities:
Employee and director stock options (in shares)									10,438	9,090	6,993
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)									5,639,863	5,621,138	5,602,377
Basic net income per common share (in dollars per share)	$ 1.29	$ 1.66	$ 1.08	$ 0.99	$ 0.89	$ 1.76	$ 1.07	$ 0.91	$ 5.01	$ 4.64	$ 3.52
Diluted net income per common share (in dollars per share)	$ 1.28	$ 1.65	$ 1.08	$ 0.98	$ 0.89	$ 1.76	$ 1.07	$ 0.91	5.00	4.64	3.51
Cash dividends per share of common stock (in dollars per share)									$ 0.64	5.89	$ 0.54
Special cash dividends per share of common stock (in dollars per share)										$ 5.35
Options
Effect of dilutive securities:
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities									5,550	8,700	9,900