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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income	$ 11,574,000	$ 10,123,000	$ 9,446,000	$ 10,881,000	$ 7,979,000	$ 8,337,000	$ 7,343,000	$ 7,615,000	$ 42,024,000	$ 31,274,000	$ 31,913,000
Adjustments to reconcile net income to net cash from operating activities:
Stock-based compensation expense									2,410,000	1,981,000	1,459,000
Stock grants to directors for retainer fees									441,000	363,000	327,000
Gain on trust preferred securities repurchase									0	0	(2,970,000)
Other assets									(2,193,000)	(354,000)	(648,000)
Change in other liabilities									11,029,000	(943,000)	(1,046,000)
Cash flows from investing activities
Net cash for investing activities									(210,480,000)	(191,472,000)	(96,561,000)
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises									108,000	318,000	978,000
Employee stock purchase plan									52,000	46,000	36,000
Dividend reinvestment plan									1,165,000	1,576,000	1,601,000
Repurchase of common stock									(5,943,000)	0	(3,732,000)
Payment of cash dividends to common shareholders									(27,500,000)	(12,046,000)	(18,731,000)
Repurchase of trust preferred securities									0	0	(8,030,000)
Net cash for financing activities									23,996,000	169,312,000	155,664,000
Net change in cash and cash equivalents									(124,747,000)	16,505,000	93,705,000
Parent Company [Member]
Cash flows from operating activities
Net income									42,024,000	31,274,000	31,913,000
Adjustments to reconcile net income to net cash from operating activities:
Equity in undistributed net income of subsidiary									(14,005,000)	(17,158,000)	(3,999,000)
Stock-based compensation expense									2,410,000	1,981,000	1,459,000
Stock grants to directors for retainer fees									441,000	363,000	327,000
Gain on trust preferred securities repurchase									0	0	(2,970,000)
Other assets									1,384,000	(230,000)	387,000
Change in other liabilities									(187,000)	(677,000)	78,000
Net cash from operating activities									32,067,000	15,553,000	27,195,000
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									108,000	318,000	978,000
Employee stock purchase plan									52,000	46,000	36,000
Dividend reinvestment plan									1,165,000	1,576,000	1,601,000
Repurchase of common stock									(5,943,000)	0	(3,732,000)
Payment of cash dividends to common shareholders									(27,500,000)	(12,046,000)	(18,731,000)
Repurchase of trust preferred securities									0	0	(8,030,000)
Net cash for financing activities									(32,118,000)	(10,106,000)	(27,878,000)
Net change in cash and cash equivalents									(51,000)	5,447,000	(683,000)
Cash and cash equivalents at beginning of period				$ 9,704,000				$ 4,257,000	9,704,000	4,257,000	4,940,000
Cash and cash equivalents at end of period	$ 9,653,000				$ 9,704,000				$ 9,653,000	$ 9,704,000	$ 4,257,000