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Note 12 - Regulatory Matters (Details Textual) - USD ($) |  |  |  |  |  |  |  | 3 Months Ended | 6 Months Ended | 18 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 21, 2016 | Jul. 14, 2016 | Jun. 23, 2016 | Apr. 14, 2016 | Mar. 23, 2016 | Jan. 26, 2016 | Jan. 14, 2016 | Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Apr. 19, 2016 | Dec. 31, 2015 | Jan. 30, 2015 | Feb. 29, 2012 | Dec. 31, 2009
Fund [Member] | Mercantile Bank Capital TrustI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Securities of Subsidiary Trust |  |  |  |  |  | $ 11,000,000 |  |  |  |  |  |  |  |  |  |  |
Mercantile Bank Capital TrustI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  | $ 1,800,000 |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Repurchase of Trust Preferred Securities Per Diluted Share |  |  |  |  |  | $ 0.11 |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared |  | $ 0.17 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Cash Paid | $ 0.17 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared |  |  |  | $ 0.16 |  |  | $ 0.16 |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Cash Paid |  |  | $ 0.16 |  | $ 0.16 |  |  | $ 0.16 | $ 0.14 | $ 0.32 | $ 0.28 |  |  |  |  |  |
Trust Preferred Securities Included in Tier One Capital |  |  |  |  |  |  |  | $ 42,200,000 |  | $ 42,200,000 |  | $ 42,200,000 |  | $ 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 OneC apital |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000,000
Preferred Securities of Subsidiary Trust |  |  |  |  |  |  |  | 42,200,000 |  | $ 42,200,000 |  | 42,200,000 |  | 53,100,000 |  |  |
Payments for Repurchase of Trust Preferred Securities |  |  |  |  |  |  |  |  |  | 8,030,000 | $ 0 |  |  |  |  |  |
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust |  |  |  |  |  |  |  | 44,494,000 |  | 44,494,000 |  | $ 44,494,000 |  | $ 55,154,000 |  | $ 32,000,000 |
Gain (Loss) on Repurchase of Trust Preferred Securities |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 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 |  |  |  |  |  | $ 0.11 |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Tier One Capital and Total Risk Based Capital Ratio Basis Point |  |  |  |  |  | (0.35%) |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Capital Ratio Basis Point |  |  |  |  |  | (0.35%) |  |  |  |  |  |  |  |  |  |  |
Stock Repurchase Program, Authorized Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,000,000 |  |
Stock Repurchase Program Additional Authorized Amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000 |  |  |  |
Stock Repurchased During Period, Shares |  |  |  |  |  |  |  |  |  | 168,000 |  | 956,000 |  |  |  |  |
Treasury Stock Acquired, Average Cost Per Share |  |  |  |  |  |  |  |  |  | $ 22.23 |  | $ 20.38 |  |  |  |  |
Stock Repurchased During Period, Value |  |  |  |  |  |  |  |  |  | $ 3,700,000 |  | $ 19,500,000 |  |  |  |  |

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- Definition Amount of gain (loss), 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 Represents the gain per diluted share on repurchase of trust preferred securities. + References No definition available. + Details
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- Definition The base point increase or decrease in capital ratio. + References No definition available. + Details
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- Definition The increase or decrease in the company's tangible book value per share. + References No definition available. + Details
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- Definition The basis point increase or decrease in the company's regulatory tier 1 capital and total risk-based capital ratios. + 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 The percentage of trust preferred securities face amount to be paid. + References No definition available. + Details
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- Definition Trust preferred securities possesses characteristics of both equity and debt issues and are generally issued by bank holding companies. + References No definition available. + Details
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- Definition Trust preferred securities held by the trust that are included in Tier One Capital and subject to certain limitations. + References No definition available. + Details
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- Definition Aggregate dividends paid during the period for each share of common stock outstanding. + References Reference 1: 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 + Details
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- Definition Aggregate dividends declared during the period for each share of common stock outstanding. + References Reference 1: 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 + 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 Amount of stock repurchase plan authorized. + References No definition available. + 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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- Definition Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate 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 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 2: 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 3: 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 Reference 4: 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 + Details
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- Definition Total cost of shares repurchased divided by the total number of shares repurchased. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655 + Details
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