Note 12 - Stock-based Compensation - Summary of Information Related to Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 27.66
Average remaining option term (years) (Year)	5 years	6 years 255 days	6 years 109 days
First Bank Plans [Member]
Minimum exercise price (in dollars per share)	$ 6.89	$ 5.19	$ 5.19
Maximum exercise price (in dollars per share)	$ 8.60	$ 16	$ 22
Average remaining option term (years) (Year)	1 year 255 days	1 year 292 days	2 years 255 days