Note 15 - Stock-based Compensation Plans - Summary of Nonvested Restricted Shares (Details) - Restricted Stock [Member] - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Nonvested at beginning of year (in shares)	32,650
Nonvested at beginning of year (in dollars per share)	$ 53.93
Granted, number of shares (in shares)	0
Granted, weight average grant date fair value (in dollars per share)	$ 0
Vested, number of shares (in shares)	(10,300)	(9,350)	(6,900)
Vested, weight average grant date fair value (in dollars per share)	$ 51.69
Forfeited, number of shares (in shares)	0
Forfeited, weight average grant date fair value (in dollars per share)	$ 0
Nonvested, number of shares (in shares)	22,350	32,650
Nonvested, weight average grant date fair value (in dollars per share)	$ 54.96	$ 53.93