Note 21 - Regulatory Matters (Details Textual) - USD ($)												9 Months Ended	12 Months Ended			23 Months Ended
	Mar. 22, 2017	Jan. 12, 2017	Dec. 21, 2016	Oct. 13, 2016	Sep. 21, 2016	Jul. 14, 2016	Jun. 23, 2016	Apr. 14, 2016	Mar. 23, 2016	Jan. 26, 2016	Jan. 14, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Jan. 01, 2019	Apr. 19, 2016	Jan. 30, 2015	Feb. 29, 2012	Dec. 31, 2009
Capital Conservation Buffer													2.50%			2.50%
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval													$ 54,600,000			$ 54,600,000
Common Stock, Dividends, Per Share, Declared				$ 0.17		$ 0.17		$ 0.16			$ 0.16
Stock Repurchase Program, Authorized Amount																			$ 20,000,000
Stock Repurchase Program Additional Authorized Amount																		$ 15,000,000
Stock Repurchased During Period, Shares													167,878			956,419
Stock Repurchased During Period, Value													$ 3,700,000			$ 19,500,000
Treasury Stock Acquired, Average Cost Per Share													$ 22.23			$ 20.38
Trust Preferred Securities Included in Tier One Capital													$ 42,800,000	$ 53,100,000		$ 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 OneC apital																					$ 15,000,000,000
Preferred Securities of Subsidiary Trust													42,800,000	53,100,000		42,800,000
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		44,835,000				$ 32,000,000
Gain (Loss) on Repurchase of Trust Preferred Securities													2,970,000	$ 0	$ 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%)
Dividends, Cash										$ 9,100,000
Increase (Decrease) in Capital Ratio Basis Point										(0.35%)
Common Stock, Dividends, Per Share, Cash Paid					$ 0.17		$ 0.16		$ 0.16
Mercantile Bank Capital Trust I [Member]
Preferred Securities of Subsidiary Trust										$ 32,000,000			$ 21,000,000			$ 21,000,000
Payments for Repurchase of Trust Preferred Securities										11,000,000
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust										$ 21,000,000
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										$ 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
Special Dividends [Member]
Common Stock, Dividends, Per Share, Declared				$ 0.50
Common Stock, Dividends, Per Share, Cash Paid			$ 0.50
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