Note 24 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Balance Sheets (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ASSETS
Other assets	$ 54,267,000	$ 44,599,000
Total assets	5,257,749,000	4,437,344,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	4,801,190,000	3,995,790,000
Subordinated debentures	48,244,000	47,563,000
Subordinated notes	73,646,000	0
Shareholders’ equity	456,559,000	441,554,000	$ 416,561,000	$ 375,249,000
Total liabilities and shareholders’ equity	5,257,749,000	4,437,344,000
Parent Company [Member]
ASSETS
Cash and cash equivalents	14,570,000	12,746,000	$ 6,045,000	$ 9,653,000
Investment in bank subsidiary	545,691,000	458,830,000
Other assets	19,576,000	18,934,000
Total assets	579,837,000	490,510,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	1,388,000	1,393,000
Subordinated debentures	48,244,000	47,563,000
Subordinated notes	73,646,000	0
Shareholders’ equity	456,559,000	441,554,000
Total liabilities and shareholders’ equity	$ 579,837,000	$ 490,510,000