Note 24 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Balance Sheets (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 29, 2012
ASSETS
Other assets	$ 28,976,000	$ 30,146,000
Total assets	3,286,704,000	3,082,571,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	2,920,834,000	2,741,760,000
Subordinated debentures	45,517,000	44,835,000			$ 32,000,000
Shareholders’ equity	365,870,000	340,811,000	$ 333,804,000	$ 328,138,000
Total liabilities and shareholders’ equity	3,286,704,000	3,082,571,000
Parent Company [Member]
ASSETS
Cash and cash equivalents	9,704,000	4,257,000	$ 4,940,000	$ 1,441,000
Investment in bank subsidiary	383,941,000	365,291,000
Other assets	20,435,000	20,665,000
Total assets	414,080,000	390,213,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	2,693,000	4,567,000
Subordinated debentures	45,517,000	44,835,000
Shareholders’ equity	365,870,000	340,811,000
Total liabilities and shareholders’ equity	$ 414,080,000	$ 390,213,000