Note 11 - Stock-based Compensation - Summary of Stock Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)	0	0	0
Exercised (in shares)	(753)
Mercantile Plans [Member]
Outstanding at beginning of year (in shares)	10,700	18,900	30,908
Outstanding at beginning of year, weighted average exercise price (in dollars per share)	$ 30.25	$ 23.92	$ 18.67
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)	0	0	0
Exercised (in shares)	(1,000)	(8,200)	(12,008)
Exercised, weighted average exercise price (in dollars per share)	$ 5.19	$ 15.69	$ 10.38
Forfeited or expired (in shares)	0	0	0
Outstanding at end of year (in shares)	9,700	10,700	18,900
Outstanding at end of year, weighted average exercise price (in dollars per share)	$ 32.83	$ 30.25	$ 23.92
Options exercisable at year-end (in shares)	9,700	10,700	18,900
Options exercisable at year-end, weighted average exercise price (in dollars per share)	$ 32.83	$ 30.25	$ 23.92