Note 24 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Statements of Income (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income
Interest and dividends from subsidiaries	$ 37,046,000	$ 35,888,000	$ 35,774,000	$ 34,786,000	$ 37,620,000	$ 35,594,000	$ 37,164,000	$ 37,935,000	$ 143,494,000	$ 148,313,000	$ 158,337,000
Expenses
Interest expense									19,432,000	26,067,000	33,803,000
Federal income tax benefit									14,695,000	10,710,000	11,004,000
Net income	$ 11,639,000	$ 15,051,000	$ 18,091,000	$ 14,240,000	$ 14,082,000	$ 10,686,000	$ 8,697,000	$ 10,673,000	59,021,000	44,138,000	49,456,000
Comprehensive income									49,806,000	45,929,000	61,390,000
Parent Company [Member]
Income
Interest and dividends from subsidiaries									39,058,000	32,588,000	22,246,000
Total income									39,058,000	32,588,000	22,246,000
Expenses
Interest expense									1,934,000	2,268,000	3,153,000
Other operating expenses									5,831,000	4,441,000	4,804,000
Total expenses									7,765,000	6,709,000	7,957,000
Income before income tax benefit and equity in undistributed net income of subsidiary									31,293,000	25,879,000	14,289,000
Federal income tax benefit									(1,653,000)	(1,173,000)	(1,718,000)
Equity in undistributed net income of subsidiary									26,075,000	17,086,000	33,449,000
Net income									59,021,000	44,138,000	49,456,000
Comprehensive income									$ 49,806,000	$ 45,929,000	$ 61,390,000