Note 23 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Statement of Cash Flows (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 14,082,000	$ 10,686,000	$ 8,697,000	$ 10,673,000	$ 13,317,000	$ 12,600,000	$ 11,715,000	$ 11,824,000	$ 44,138,000	$ 49,456,000	$ 42,024,000
Adjustments to reconcile net income to net cash from operating activities:
Stock-based compensation expense									2,325,000	2,931,000	2,410,000
Stock grants to directors for retainer fees									394,000	375,000	441,000
Other assets									(10,892,000)	(7,636,000)	(2,193,000)
Accrued interest and other liabilities									1,895,000	(2,818,000)	11,029,000
Cash flows from investing activities
Net cash for investing activities									(398,806,000)	(92,838,000)	(210,480,000)
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises									3,000	128,000	108,000
Employee stock purchase plan									49,000	50,000	52,000
Dividend reinvestment plan									814,000	729,000	1,165,000
Repurchases of common stock									(6,591,000)	(7,183,000)	(5,943,000)
Payment of cash dividends to common shareholders									(17,930,000)	(17,108,000)	(27,500,000)
Net cash for financing activities									753,204,000	206,448,000	23,996,000
Parent Company [Member]
Cash flows from operating activities
Net income									44,138,000	49,456,000	42,024,000
Adjustments to reconcile net income to net cash from operating activities:
Equity in undistributed net income of subsidiary									(17,086,000)	(33,449,000)	(14,005,000)
Stock-based compensation expense									2,325,000	2,931,000	2,410,000
Stock grants to directors for retainer fees									394,000	375,000	441,000
Other assets									(227,000)	2,387,000	1,384,000
Accrued interest and other liabilities									812,000	(1,924,000)	(187,000)
Net cash from operating activities									30,356,000	19,776,000	32,067,000
Cash flows from investing activities
Net capital investment into subsidiaries									0	0	0
Net cash for investing activities									0	0	0
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises									3,000	128,000	108,000
Employee stock purchase plan									49,000	50,000	52,000
Dividend reinvestment plan									814,000	729,000	1,165,000
Repurchases of common stock									(6,591,000)	(7,183,000)	(5,943,000)
Payment of cash dividends to common shareholders									(17,930,000)	(17,108,000)	(27,500,000)
Net cash for financing activities									(23,655,000)	(23,384,000)	(32,118,000)
Net change in cash and cash equivalents									6,701,000	(3,608,000)	(51,000)
Cash and cash equivalents at beginning of period				$ 6,045,000				$ 9,653,000	6,045,000	9,653,000	9,704,000
Cash and cash equivalents at end of period	$ 12,746,000				$ 6,045,000				$ 12,746,000	$ 6,045,000	$ 9,653,000