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Note 20 - Subordinated Debentures (Details Textual)			12 Months Ended
	Jan. 26, 2016 USD ($)	Jun. 01, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Feb. 29, 2012 USD ($)
Number of Business Trusts			5
Business Combination Debt Assumed Number of Trusts			4
Fair Value Discount of Subordinated Debentures Related to Assumed Business Trusts at Time af Merger		$ 15,000,000
Amortization of Fair Value Discount Related to Subordinated Debentures of Assumed Business Trusts		$ 700,000
Amortization Period of Fair Value Discount Related to Subordinated Debentures of Assumed Business Trusts		21 years 182 days
Preferred Securities of Subsidiary Trust			$ 42,800,000	$ 53,100,000
Payments for Repurchase of Trust Preferred Securities			8,030,000	0	$ 0
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust			44,835,000	$ 55,154,000		$ 32,000,000
Mercantile Bank Capital Trust I [Member]
Preferred Securities of Subsidiary Trust	$ 32,000,000		$ 21,000,000
Payments for Repurchase of Trust Preferred Securities	11,000,000
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust	21,000,000
Fund [Member] | Mercantile Bank Capital Trust I [Member]
Preferred Securities of Subsidiary Trust	$ 11,000,000

X

- Definition

Represents the annual amortization of the fair value discount related to subordinated debentures of assumed business trusts over a period of time.

+ References

No definition available.

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- Definition

Represents the amortization period of the fair value discount related to subordinated debentures of assumed business trusts.

+ References

No definition available.

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- Definition

The number of trusts for which debt assumed in a business combination are payable to.

+ References

No definition available.

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- Definition

Represents the fair value discount of subordinated debentures related to assumed business trusts at the time of the merger.

+ References

No definition available.

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- Definition

Represents the Company's business trusts.

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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.

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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

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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

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