Note 10 - Stock-based Compensation - Summary of Stock Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Exercised (in shares)		(1,355)	(2,000)
Mercantile Plans [Member]
Outstanding at beginning of year (in shares)	5,000
Outstanding at beginning of year, weighted average exercise price (in dollars per share)	$ 36.22
Granted (in shares)	0
Exercised (in shares)	0
Forfeited or expired (in shares)	(5,000)
Forfeited or expired, weighted average exercise price (in dollars per share)	$ 34.89
Outstanding at end of year (in shares)	0	5,000
Outstanding at end of year, weighted average exercise price (in dollars per share)	$ 0	$ 36.22
Options exercisable at year end (in shares)	0
Options exercisable at year-end, weighted average exercise price (in dollars per share)	$ 0