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