Note 12 - Stock-based Compensation - Summary of Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Mercantile Plans [Member]
Minimum exercise price (in dollars per share)	$ 22.15	$ 22.15	$ 6.21
Maximum exercise price (in dollars per share)	$ 36.22	$ 27.66	$ 35.88
Outstanding Number (in shares)	16,808
Average remaining option term (years) (Year)	6 years 255 days	6 years 109 days	2 years
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 29.17
Exercisable Number (in shares)	5,488
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 22.15
Mercantile Plans [Member] | Exercise Price Range 1 [Member]
Minimum exercise price (in dollars per share)	20
Maximum exercise price (in dollars per share)	$ 24
Outstanding Number (in shares)	5,488
Average remaining option term (years) (Year)	4 years 328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 22.15
Exercisable Number (in shares)	5,488
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)	4,820
Average remaining option term (years) (Year)	5 years 328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 27.66
Exercisable Number (in shares)	0
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 0
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 (years) (Year)	6 years 328 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 36.22
Exercisable Number (in shares)	0
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 0
First Bank Plans [Member]
Minimum exercise price (in dollars per share)	5.19	$ 5.19	$ 5.19
Maximum exercise price (in dollars per share)	$ 16	$ 22	$ 24.46
Outstanding Number (in shares)	44,275
Average remaining option term (years) (Year)	1 year 292 days	2 years 255 days	3 years 36 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 9.86
Exercisable Number (in shares)	44,275
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 9.86
First Bank Plans [Member] | Exercise Price Range 1 [Member]
Minimum exercise price (in dollars per share)	4
Maximum exercise price (in dollars per share)	$ 8
Outstanding Number (in shares)	23,975
Average remaining option term (years) (Year)	2 years 73 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 6.73
Exercisable Number (in shares)	23,975
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 6.73
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)	$ 12
Outstanding Number (in shares)	6,700
Average remaining option term (years) (Year)	2 years 255 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 8.60
Exercisable Number (in shares)	6,700
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 8.60
First Bank Plans [Member] | Exercise Price Range 3 [Member]
Minimum exercise price (in dollars per share)	12.01
Maximum exercise price (in dollars per share)	$ 16
Outstanding Number (in shares)	13,600
Average remaining option term (years) (Year)	255 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 16
Exercisable Number (in shares)	13,600
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 16