Note 24 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Statement of Cash Flows (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
Cash flows from operating activities
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
Adjustments to reconcile net income to net cash from operating activities:
Stock-based compensation expense									3,784,000	2,325,000	2,931,000
Stock grants to directors for retainer fees									344,000	394,000	375,000
Other assets									(16,279,000)	(10,892,000)	(7,636,000)
Accrued interest and other liabilities									335,000	1,895,000	(2,818,000)
Cash flows from investing activities
Net cash for investing activities									(495,507,000)	(398,806,000)	(92,838,000)
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises									50,000	3,000	128,000
Employee stock purchase plan									48,000	49,000	50,000
Dividend reinvestment plan									877,000	814,000	729,000
Net proceeds from subordinated notes issuance									73,635,000	0	0
Repurchases of common stock									(21,380,000)	(6,591,000)	(7,183,000)
Payment of cash dividends to common shareholders									(18,524,000)	(17,930,000)	(17,108,000)
Net cash for financing activities									780,088,000	753,204,000	206,448,000
Parent Company [Member]
Cash flows from operating activities
Net income									59,021,000	44,138,000	49,456,000
Adjustments to reconcile net income to net cash from operating activities:
Equity in undistributed net income of subsidiary									(26,075,000)	(17,086,000)	(33,449,000)
Stock-based compensation expense									3,784,000	2,325,000	2,931,000
Stock grants to directors for retainer fees									344,000	394,000	375,000
Other assets									49,000	(227,000)	2,387,000
Accrued interest and other liabilities									(5,000)	812,000	(1,924,000)
Net cash from operating activities									37,118,000	30,356,000	19,776,000
Cash flows from investing activities
Net capital investment into subsidiaries									(70,000,000)	0	0
Net cash for investing activities									(70,000,000)	0	0
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises									50,000	3,000	128,000
Employee stock purchase plan									48,000	49,000	50,000
Dividend reinvestment plan									877,000	814,000	729,000
Net proceeds from subordinated notes issuance									73,635,000	0	0
Repurchases of common stock									(21,380,000)	(6,591,000)	(7,183,000)
Payment of cash dividends to common shareholders									(18,524,000)	(17,930,000)	(17,108,000)
Net cash for financing activities									34,706,000	(23,655,000)	(23,384,000)
Net change in cash and cash equivalents									1,824,000	6,701,000	(3,608,000)
Cash and cash equivalents at beginning of period				$ 12,746,000				$ 6,045,000	12,746,000	6,045,000	9,653,000
Cash and cash equivalents at end of period	$ 14,570,000				$ 12,746,000				$ 14,570,000	$ 12,746,000	$ 6,045,000