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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 13,317,000	$ 12,600,000	$ 11,715,000	$ 11,824,000	$ 11,574,000	$ 10,123,000	$ 9,446,000	$ 10,881,000	$ 49,456,000	$ 42,024,000	$ 31,274,000
Adjustments to reconcile net income to net cash from operating activities:
Stock-based compensation expense									2,931,000	2,410,000	1,981,000
Stock grants to directors for retainer fees									375,000	441,000	363,000
Other assets									(7,636,000)	(2,193,000)	(354,000)
Change in other liabilities									(2,818,000)	11,029,000	(943,000)
Cash flows from investing activities
Net cash for investing activities									(92,838,000)	(210,480,000)	(191,472,000)
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises									128,000	108,000	318,000
Employee stock purchase plan									50,000	52,000	46,000
Dividend reinvestment plan									729,000	1,165,000	1,576,000
Repurchases of common stock									(7,183,000)	(5,943,000)	0
Payment of cash dividends to common shareholders									(17,108,000)	(27,500,000)	(12,046,000)
Net cash for financing activities									206,448,000	23,996,000	169,312,000
Net change in cash and cash equivalents									158,377,000	(124,747,000)	16,505,000
Parent Company [Member]
Cash flows from operating activities
Net income									49,456,000	42,024,000	31,274,000
Adjustments to reconcile net income to net cash from operating activities:
Equity in undistributed net income of subsidiary									(33,449,000)	(14,005,000)	(17,158,000)
Stock-based compensation expense									2,931,000	2,410,000	1,981,000
Stock grants to directors for retainer fees									375,000	441,000	363,000
Other assets									2,387,000	1,384,000	(230,000)
Change in other liabilities									(1,924,000)	(187,000)	(677,000)
Net cash from operating activities									19,776,000	32,067,000	15,553,000
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									128,000	108,000	318,000
Employee stock purchase plan									50,000	52,000	46,000
Dividend reinvestment plan									729,000	1,165,000	1,576,000
Repurchases of common stock									(7,183,000)	(5,943,000)	0
Payment of cash dividends to common shareholders									(17,108,000)	(27,500,000)	(12,046,000)
Net cash for financing activities									(23,384,000)	(32,118,000)	(10,106,000)
Net change in cash and cash equivalents									(3,608,000)	(51,000)	5,447,000
Cash and cash equivalents at beginning of period				$ 9,653,000				$ 9,704,000	9,653,000	9,704,000	4,257,000
Cash and cash equivalents at end of period	$ 6,045,000				$ 9,653,000				$ 6,045,000	$ 9,653,000	$ 9,704,000