Note 21 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income
Interest and dividends from subsidiaries	$ 271,358	$ 181,839	$ 143,494
Expenses
Interest expense	77,813	23,595	19,432
Federal income tax benefit	20,482	14,727	14,695
Net income	82,217	61,063	59,021
Other comprehensive income (loss)	14,854	(61,612)	(9,215)
Comprehensive income (loss)	97,071	(549)	49,806
Parent Company [Member]
Income
Interest and dividends from subsidiaries	26,660	26,056	39,058
Total income	26,660	26,056	39,058
Expenses
Interest expense	8,091	6,104	1,934
Other operating expenses	5,674	5,645	5,831
Total expenses	13,765	11,749	7,765
Income before income tax benefit and equity in undistributed net income of subsidiary	12,895	14,307	31,293
Federal income tax benefit	(2,858)	(2,535)	(1,653)
Equity in undistributed net income of subsidiary	66,464	44,221	26,075
Net income	82,217	61,063	59,021
Other comprehensive income (loss)	14,854	(61,612)	(9,215)
Comprehensive income (loss)	$ 97,071	$ (549)	$ 49,806