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Note 12 - Regulatory Matters (Details) - USD ($) |  |  |  |  |  | 3 Months Ended | 14 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 23, 2016 | Apr. 14, 2016 | Mar. 23, 2016 | Jan. 26, 2016 | Jan. 14, 2016 | Mar. 31, 2016 | Mar. 31, 2015 | Mar. 31, 2016 | Apr. 19, 2016 | Dec. 31, 2015 | Jan. 30, 2015 | Feb. 29, 2012 | Dec. 31, 2009
Note 12 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Trust Preferred Securities Included in Tier One Capital |  |  |  |  |  | $ 42,300,000 |  | $ 42,300,000 |  | $ 53,100,000 |  |  |
Maximum Restricted Core Element Allowed in Tier One Capital, Percent |  |  |  |  |  | 25.00% |  |  |  |  |  |  |
Maximum Level of Consolidated Aggregate Assets Allowing for Inclusion of Trust Preferred Securities in Tier One Capital |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000,000
Preferred Securities of Subsidiary Trust |  |  |  |  |  | $ 42,300,000 |  | 42,300,000 |  | 53,100,000 |  |  |
Payments for Repurchase of Trust Preferred Securities |  |  |  |  |  | 8,030,000 | $ 0 |  |  |  |  |  |
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust |  |  |  |  |  | 44,324,000 |  | $ 44,324,000 |  | $ 55,154,000 |  | $ 32,000,000 |
Gain (Loss) on Repurchase of Trust Preferred Securities, After Tax |  |  |  |  |  | $ 2,970,000 | $ 0 |  |  |  |  |  |
Increase (Decrease) in Tangible Equity to Tangible Assets Ratio, Basis Point |  |  |  | 0.09% |  |  |  |  |  |  |  |  |
Increase (Decrease) in Tangible Book Value Per Share (in Dollars per share) |  |  |  | $ 0.11 |  |  |  |  |  |  |  |  |
Increase (Decrease) in Tier One Capital and Total Risk-based Capital Ratio, Basis Point |  |  |  | (0.35%) |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared (in Dollars per share) |  |  |  |  | $ 0.16 |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Cash Paid (in Dollars per share) |  |  | $ 0.16 |  |  | $ 0.16 | $ 0.14 |  |  |  |  |  |
Stock Repurchase Program, Authorized Amount |  |  |  |  |  |  |  |  |  |  | $ 20,000,000 |  |
Stock Repurchased During Period, Shares (in Shares) |  |  |  |  |  | 148,000 |  | 936,000 |  |  |  |  |
Treasury Stock Acquired, Average Cost Per Share (in Dollars per share) |  |  |  |  |  | $ 22.07 |  | $ 20.32 |  |  |  |  |
Stock Repurchased During Period, Value |  |  |  |  |  | $ 3,300,000 |  | $ 19,000,000 |  |  |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Note 12 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Cash Paid (in Dollars per share) | $ 0.16 |  |  |  |  |  |  |  |  |  |  |  |
Mercantile Bank Capital Trust I [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Note 12 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Securities of Subsidiary Trust |  |  |  | $ 32,000,000 |  |  |  |  |  |  |  |  |
Payments for Repurchase of Trust Preferred Securities |  |  |  | 11,000,000 |  |  |  |  |  |  |  |  |
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust |  |  |  | $ 21,000,000 |  |  |  |  |  |  |  |  |
Percentage of Trust Preferred Securities Face Amount Paid |  |  |  | 73.00% |  |  |  |  |  |  |  |  |
Discount Rate of Trust Preferred Securities |  |  |  | 27.00% |  |  |  |  |  |  |  |  |
Gain (Loss) on Repurchase of Trust Preferred Securities, before Tax |  |  |  | $ 3,000,000 |  |  |  |  |  |  |  |  |
Gain (Loss) on Repurchase of Trust Preferred Securities, After Tax |  |  |  | $ 1,800,000 |  |  |  |  |  |  |  |  |
Gain (Loss) on Repurchase of Trust Preferred Securities, Per Diluted Share (in Dollars per share) |  |  |  | $ 0.11 |  |  |  |  |  |  |  |  |
Mercantile Bank Capital Trust I [Member] | Fund [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Note 12 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Securities of Subsidiary Trust |  |  |  | $ 11,000,000 |  |  |  |  |  |  |  |  |
Bank [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Note 12 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Capital Ratio, Basis Point |  |  |  | (0.35%) |  |  |  |  |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Note 12 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared (in Dollars per share) |  | $ 0.16 |  |  |  |  |  |  |  |  |  |  |
Stock Repurchase Program, Additional Authorized Amount |  |  |  |  |  |  |  |  | $ 15,000,000 |  |  |  |

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- Definition Amount of gain (loss), after tax, from the difference between the repurchase price of the trust preferred securities initially issued by the entity or its subsidiaries and the net carrying amount of the debt at the time of its repurchase. + References No definition available. + Details
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- Definition Amount of gain (loss), pre-tax, from the difference between the repurchase price of the trust preferred securities initially issued by the entity or its subsidiaries and the net carrying amount of the debt at the time of its repurchase. + References No definition available. + Details
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- Definition The maximum level of consolidated aggregate assets allowing for inclusion of trust preferred securities in Tier 1 Capital under the Dodd-Frank Act. + References No definition available. + Details
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- Definition Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition The cash outflow to reacquire preferred stocks issued by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3291-108585 + Details
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- Definition Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 + Details
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