Note 12 - Stock-based Compensation - Summary of Stock Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0
Mercantile Plans [Member]
Outstanding at beginning of year (in shares)	15,808	16,808	11,308
Outstanding at beginning of year (in dollars per share)	$ 29.62	$ 29.17	$ 24.50
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0	6,500
Granted (in dollars per share)	$ 0		$ 36.22
Exercised (in shares)	(2,108)	(1,000)	(1,000)
Exercised (in dollars per share)	$ 23.77	$ 22.15	$ 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)	13,700	15,808	16,808
Outstanding at end of year (in dollars per share)	$ 30.51	$ 29.62	$ 29.17
Options exercisable at year-end (in shares)	13,700	9,308	5,488
Options exercisable at year-end (in dollars per share)	$ 30.51	$ 25	$ 22.15
First Bank Plans [Member]
Outstanding at beginning of year (in shares)	15,100	44,275	123,887
Outstanding at beginning of year (in dollars per share)	$ 7.20	$ 9.86	$ 12.64
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0	0
Granted (in dollars per share)	$ 0	$ 0	$ 0
Exercised (in shares)	(9,900)	(27,675)	(71,711)
Exercised (in dollars per share)	$ 7.53	$ 11.50	$ 13.33
Forfeited or expired (in shares)	0	(1,500)	(7,901)
Forfeited or expired (in dollars per share)		$ 6.33	$ 22
Outstanding at end of year (in shares)	5,200	15,100	44,275
Outstanding at end of year (in dollars per share)	$ 6.57	$ 7.20	$ 9.86
Options exercisable at year-end (in shares)	5,200	15,100	44,275
Options exercisable at year-end (in dollars per share)	$ 6.57	$ 7.20	$ 9.86