Note 25 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements (Details) - Condensed Statement of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net income	$ 17,331,000	$ 17,033,000	$ 12,535,000
Adjustments to reconcile net income to net cash from operating activities:
Stock-based compensation expense	714,000	473,000	54,000
Change in other assets	1,795,000	8,465,000	(1,805,000)
Net cash from operating activities	14,414,000	28,682,000	17,031,000
Cash flows from investing activities
Net cash for investing activities	64,763,000	(12,419,000)	66,730,000
Cash flows from financing activities
Repurchase of preferred stock	0	0	(21,000,000)
Repurchase of common stock warrant	0	0	(7,465,000)
Stock option exercises, net of cashless exercises	282,000	289,000	227,000
Employee stock purchase plan	23,000	19,000	39,000
Dividend reinvestment plan	209,000	33,000	14,000
Stock grants to directors for retainer fees	155,000	0	0
Cash dividends on preferred stock	0	0	(496,000)
Cash dividends on common stock	(24,464,000)	(3,889,000)	(774,000)
Net cash for financing activities	(53,404,000)	(5,301,000)	(24,130,000)
Net change in cash and cash equivalents	25,773,000	10,962,000	59,631,000
Parent Company [Member]
Cash flows from operating activities
Net income	17,331,000	17,033,000	12,535,000
Adjustments to reconcile net income to net cash from operating activities:
Equity in undistributed (income) loss of subsidiary	(9,131,000)	(14,461,000)	18,730,000
Stock-based compensation expense	714,000	473,000	54,000
Change in other assets	(8,163,000)	3,244,000	(3,006,000)
Change in other liabilities	21,979,000	(708,000)	1,073,000
Net cash from operating activities	22,730,000	5,581,000	29,386,000
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
Repurchase of preferred stock	0	0	(21,000,000)
Repurchase of common stock warrant	0	0	(7,465,000)
Stock option exercises, net of cashless exercises	282,000	289,000	227,000
Employee stock purchase plan	23,000	19,000	39,000
Dividend reinvestment plan	209,000	33,000	14,000
Stock grants to directors for retainer fees	155,000	0	0
Cash dividends on preferred stock	0	0	(496,000)
Cash dividends on common stock	(24,464,000)	(3,889,000)	(774,000)
Net cash for financing activities	(23,795,000)	(3,548,000)	(29,455,000)
Net change in cash and cash equivalents	(1,065,000)	2,033,000	(69,000)
Cash and cash equivalents at beginning of period	2,506,000	473,000	542,000
Cash and cash equivalents at end of period	$ 1,441,000	$ 2,506,000	$ 473,000