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Note 12 - Stock-based Compensation - Summary of Stock Option Grants and Exercises (Details) - USD ($) $ / shares in Thousands | 12 Months Ended
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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

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- Definition Amount of tax benefit from exercise of option under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116856206&loc=SL79508275-113901 + Details
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- Definition Amount of cash inflow from exercise of option under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116856206&loc=SL79508275-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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