Note 12 - Stock-based Compensation - Summary of Stock Option Grants and Exercises (Details) - USD ($) $ / shares in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Proceeds from stock option exercises, net of cashless exercises	$ 128,000	$ 108,000	$ 318,000
Mercantile Plans [Member]
Aggregate intrinsic value of stock options exercised	43,000	17,000	15,000
Proceeds from stock option exercises, net of cashless exercises	97,000	33,000	0
Tax benefit realized from stock option exercises	0	0	0
First Bank Plans [Member]
Aggregate intrinsic value of stock options exercised	114,000	300,000	594,000
Proceeds from stock option exercises, net of cashless exercises	29,000	108,000	318,000
Tax benefit realized from stock option exercises	$ 0	$ 0	$ 208,000
Weighted average per share fair value of stock options granted (in dollars per share)	$ 0	$ 0	$ 0

Amount of tax benefit from exercise of option under share-based payment arrangement.

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Amount of cash inflow from exercise of option under share-based payment arrangement.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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