Note 12 - Stock-based Compensation - Summary of Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Mercantile Plans [Member]
Minimum exercise price (in dollars per share)	$ 22.15	$ 22.15	$ 22.15
Maximum exercise price (in dollars per share)	$ 36.22	$ 36.22	$ 36.22
Outstanding Number (in shares)	13,700
Average remaining option term (Year)	4 years 36 days	5 years	6 years 255 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 30.51
Exercisable Number (in shares)	13,700
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 30.51
Mercantile Plans [Member] | Exercise Price Range 1 [Member]
Minimum exercise price (in dollars per share)	22
Maximum exercise price (in dollars per share)	$ 23
Outstanding Number (in shares)	3,000
Average remaining option term (Year)	2 years 328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 22.15
Exercisable Number (in shares)	3,000
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 22.15
Mercantile Plans [Member] | Exercise Price Range 2 [Member]
Minimum exercise price (in dollars per share)	24.01
Maximum exercise price (in dollars per share)	$ 28
Outstanding Number (in shares)	4,200
Average remaining option term (Year)	3 years 328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 27.66
Exercisable Number (in shares)	4,200
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 27.66
Mercantile Plans [Member] | Exercise Price Range 3 [Member]
Minimum exercise price (in dollars per share)	36
Maximum exercise price (in dollars per share)	$ 40
Outstanding Number (in shares)	6,500
Average remaining option term (Year)	4 years 328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 36.22
Exercisable Number (in shares)	6,500
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 36.22
First Bank Plans [Member]
Minimum exercise price (in dollars per share)	5.19	$ 5.19	$ 5.19
Maximum exercise price (in dollars per share)	$ 8.60	$ 8.60	$ 16
Outstanding Number (in shares)	5,200
Average remaining option term (Year)	1 year 182 days	1 year 255 days	1 year 292 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 6.57
Exercisable Number (in shares)	5,200
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 6.57
First Bank Plans [Member] | Exercise Price Range 1 [Member]
Minimum exercise price (in dollars per share)	5
Maximum exercise price (in dollars per share)	$ 6
Outstanding Number (in shares)	3,100
Average remaining option term (Year)	1 year 328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 5.19
Exercisable Number (in shares)	3,100
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 5.19
First Bank Plans [Member] | Exercise Price Range 2 [Member]
Minimum exercise price (in dollars per share)	8.01
Maximum exercise price (in dollars per share)	$ 9
Outstanding Number (in shares)	2,100
Average remaining option term (Year)	328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 8.60
Exercisable Number (in shares)	2,100
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 8.60