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