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Note 12 - Stock-based Compensation - Summary of Options Outstanding (Details) - $ / shares | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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 | $ 36.22
Outstanding Number (in shares) | 9,700 |  |
Average remaining option term (Year) | 3 years 146 days | 4 years 36 days | 5 years
Outstanding Weighted Average Exercise Price (in dollars per share) | $ 32.83 |  |
Exercisable Number (in shares) | 9,700 |  |
Exercisable Weighted Average Exercise Price (in dollars per share) | $ 32.83 |  |
Mercantile Plans [Member] | Exercise Price Range 1 [Member] |  |  |
Minimum exercise price (in dollars per share) | 22 |  |
Maximum exercise price (in dollars per share) | $ 23 |  |
Outstanding Number (in shares) | 1,000 |  |
Average remaining option term (Year) | 1 year 328 days |  |
Outstanding Weighted Average Exercise Price (in dollars per share) | $ 22.15 |  |
Exercisable Number (in shares) | 1,000 |  |
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) | 27 |  |
Maximum exercise price (in dollars per share) | $ 28 |  |
Outstanding Number (in shares) | 2,200 |  |
Average remaining option term (Year) | 2 years 328 days |  |
Outstanding Weighted Average Exercise Price (in dollars per share) | $ 27.66 |  |
Exercisable Number (in shares) | 2,200 |  |
Exercisable Weighted Average Exercise Price (in dollars per share) | $ 27.66 |  |
Mercantile Plans [Member] | Exercise Price Range 3 [Member] |  |  |
Minimum exercise price (in dollars per share) | 36 |  |
Maximum exercise price (in dollars per share) | $ 37 |  |
Outstanding Number (in shares) | 6,500 |  |
Average remaining option term (Year) | 3 years 328 days |  |
Outstanding Weighted Average Exercise Price (in dollars per share) | $ 36.22 |  |
Exercisable Number (in shares) | 6,500 |  |
Exercisable Weighted Average Exercise Price (in dollars per share) | $ 36.22 |  |
First Bank Plans [Member] |  |  |
Minimum exercise price (in dollars per share) | 5.19 | $ 5.19 | $ 5.19
Maximum exercise price (in dollars per share) | $ 5.19 | $ 8.60 | $ 8.60
Outstanding Number (in shares) | 1,000 |  |
Average remaining option term (Year) | 328 days | 1 year 182 days | 1 year 255 days
Outstanding Weighted Average Exercise Price (in dollars per share) | $ 5.19 |  |
Exercisable Number (in shares) | 1,000 |  |
Exercisable Weighted Average Exercise Price (in dollars per share) | $ 5.19 |  |
First Bank Plans [Member] | Exercise Price Range 1 [Member] |  |  |
Minimum exercise price (in dollars per share) | 5 |  |
Maximum exercise price (in dollars per share) | $ 6 |  |
Outstanding Number (in shares) | 1,000 |  |
Average remaining option term (Year) | 328 days |  |
Outstanding Weighted Average Exercise Price (in dollars per share) | $ 5.19 |  |
Exercisable Number (in shares) | 1,000 |  |
Exercisable Weighted Average Exercise Price (in dollars per share) | $ 5.19 |  |

X
- Definition The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + 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 (g) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. + 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 (c)(iii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. + 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 (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + 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 (g) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding. + 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 (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices. + 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 (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (e)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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