Note 12 - Stock-based Compensation - Summary of Stock Option Grants and Exercises (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Proceeds from stock option exercises, net of cashless exercises	$ 978,000	$ 891,000	$ 282,000
Mercantile Plans [Member]
Aggregate intrinsic value of stock options exercised	6,000	36,000	11,000
Proceeds from stock option exercises, net of cashless exercises	22,000	17,000	50,000
Tax benefit realized from stock option exercises	$ 0	$ 0	$ 0
Weighted average per share fair value of stock options granted (in dollars per share)	$ 5.25	$ 4.41	$ 2.72
First Bank Plans [Member]
Aggregate intrinsic value of stock options exercised	$ 1,945,000	$ 420,000	$ 333,000
Proceeds from stock option exercises, net of cashless exercises	956,000	874,000	232,000
Tax benefit realized from stock option exercises	$ 681,000	$ 147,000	$ 116,000
Weighted average per share fair value of stock options granted (in dollars per share)	$ 0	$ 0	$ 0