Summary of Significant Accounting Policies - Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Apr. 30, 2021
Anti-dilutive securities
Common stock, authorized shares	30,000,000	15,000,000		30,000,000
Net income for basic and diluted income per common share	$ 37,045	$ 28,223	$ 26,056
Weighted-average shares for basic income per common share (in shares)	5,656,367	5,629,425	5,612,048
Effect of dilutive securities:
Employee and director stock options (in shares)	11,992	10,438	9,090
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)	5,668,359	5,639,863	5,621,138
Basic net income per common share (in dollars per share)	$ 6.55	$ 5.01	$ 4.64
Diluted net income per common share (in dollars per share)	6.54	5.00	4.64
Cash dividends per share of common stock (in dollars per share)	$ 0.64	$ 0.64	5.89
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	600	5,550	8,700