Note 12 - Stock-based Compensation - Summary of Stock Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0	0
Mercantile Plans [Member]
Outstanding at beginning of year (in shares)	13,700	15,808	16,808
Outstanding at beginning of year (in dollars per share)	$ 30.51	$ 29.62	$ 29.17
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0	0
Granted (in dollars per share)	$ 0
Exercised (in shares)	(4,000)	(2,108)	(1,000)
Exercised (in dollars per share)	$ 24.91	$ 23.77	$ 22.15
Forfeited or expired (in shares)	0	0	0
Forfeited or expired (in dollars per share)	$ 0	$ 0
Outstanding at end of year (in shares)	9,700	13,700	15,808
Outstanding at end of year (in dollars per share)	$ 32.83	$ 30.51	$ 29.62
Options exercisable at year-end (in shares)	9,700	13,700	9,308
Options exercisable at year-end (in dollars per share)	$ 32.83	$ 30.51	$ 25
First Bank Plans [Member]
Outstanding at beginning of year (in shares)	5,200	15,100	44,275
Outstanding at beginning of year (in dollars per share)	$ 6.57	$ 7.20	$ 9.86
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0	0
Granted (in dollars per share)	$ 0
Exercised (in shares)	(4,200)	(9,900)	(27,675)
Exercised (in dollars per share)	$ 6.90	$ 7.53	$ 11.50
Forfeited or expired (in shares)	0	0	(1,500)
Forfeited or expired (in dollars per share)	$ 0	$ 0	$ 6.33
Outstanding at end of year (in shares)	1,000	5,200	15,100
Outstanding at end of year (in dollars per share)	$ 5.19	$ 6.57	$ 7.20
Options exercisable at year-end (in shares)	1,000	5,200	15,100
Options exercisable at year-end (in dollars per share)	$ 5.19	$ 6.57	$ 7.20