Note 24 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Balance Sheets (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 29, 2012
ASSETS
Other assets	$ 30,146,000	$ 29,123,000
Total assets	3,082,571,000	2,903,556,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	2,741,760,000	2,569,752,000
Subordinated debentures	44,835,000	55,154,000			$ 32,000,000
Shareholders’ equity	340,811,000	333,804,000	$ 328,138,000	$ 153,325,000
Total liabilities and shareholders’ equity	3,082,571,000	2,903,556,000
Parent Company [Member]
ASSETS
Cash and cash equivalents	4,257,000	4,940,000	$ 1,441,000	$ 2,506,000
Investment in bank subsidiary	365,291,000	368,248,000
Other assets	20,665,000	20,940,000
Total assets	390,213,000	394,128,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	4,567,000	5,170,000
Subordinated debentures	44,835,000	55,154,000
Shareholders’ equity	340,811,000	333,804,000
Total liabilities and shareholders’ equity	$ 390,213,000	$ 394,128,000