Note 12 - Regulatory Matters (Details Textual) - USD ($)						3 Months Ended		26 Months Ended
	Jun. 21, 2017	Apr. 13, 2017	Mar. 22, 2017	Jan. 12, 2017	Jan. 26, 2016	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Jan. 01, 2019	Dec. 31, 2016	Apr. 19, 2016	Jan. 30, 2015	Feb. 29, 2012
Preferred Securities of Subsidiary Trust						$ 42,900,000		$ 42,900,000		$ 42,800,000
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		15,000,000,000
Trust Preferred Securities Included in Tier One Capital						$ 42,900,000		$ 42,900,000		42,800,000
Capital Conservation Buffer						2.50%		2.50%
Common Stock, Dividends, Per Share, Declared				$ 0.18
Stock Repurchase Program, Authorized Amount												$ 20,000,000
Stock Repurchase Program Additional Authorized Amount											$ 15,000,000
Stock Repurchased During Period, Shares								956,419
Stock Repurchased During Period, Value								$ 19,500,000
Treasury Stock Acquired, Average Cost Per Share								$ 20.38
Payments for Repurchase of Trust Preferred Securities						$ 0	$ 8,030,000
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust						45,006,000		$ 45,006,000		$ 44,835,000			$ 32,000,000
Gain (Loss) on Repurchase of Trust Preferred Securities						$ 0	$ 2,970,000
Dividends, Cash					$ 9,100,000
Increase (Decrease) in Capital Ratio Basis Point					(0.35%)
Common Stock, Dividends, Per Share, Cash Paid			$ 0.18			$ 0.18	$ 0.16
Mercantile Bank Capital Trust I [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
Fund [Member] | Mercantile Bank Capital Trust I [Member]
Preferred Securities of Subsidiary Trust					$ 11,000,000
Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared		$ 0.18
Scenario, Forecast [Member]
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy to Risk Weighted Assets									7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets									8.50%
Capital Required for Capital Adequacy to Risk Weighted Assets									10.50%
Common Stock, Dividends, Per Share, Cash Paid	$ 0.18