Note 12 - Regulatory Matters (Details Textual) - USD ($)										3 Months Ended		9 Months Ended		20 Months Ended
	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	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Apr. 19, 2016	Dec. 31, 2015	Jan. 30, 2015	Feb. 29, 2012	Dec. 31, 2009
Subsequent Event [Member] | Special Dividends [Member]
Common Stock, Dividends, Per Share, Declared		$ 0.50
Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared		$ 0.17
Scenario, Forecast [Member] | Special Dividends [Member]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.50
Scenario, Forecast [Member]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.17
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
Minimum [Member]
Increase (Decrease) on Regulatory Capital Ratios from Dividends Declared												(0.25%)
Maximum [Member]
Increase (Decrease) on Regulatory Capital Ratios from Dividends Declared												(0.30%)
Common Stock, Dividends, Per Share, Cash Paid			$ 0.17		$ 0.16		$ 0.16			$ 0.17	$ 0.15	$ 0.49	$ 0.43
Common Stock, Dividends, Per Share, Declared				$ 0.17		$ 0.16			$ 0.16
Trust Preferred Securities Included in Tier One Capital										$ 42,600,000		$ 42,600,000		$ 42,600,000		$ 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										42,600,000		$ 42,600,000		42,600,000		53,100,000
Payments for Repurchase of Trust Preferred Securities												8,030,000	$ 0
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust										44,665,000		44,665,000		$ 44,665,000		$ 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