Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements (Details 2) (USD $)	1 Months Ended	12 Months Ended
	Sep. 30, 2004	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities
Net income (loss)		$ 12,535,000	$ 37,485,000	$ (13,316,000)
Adjustments to reconcile net income (loss) to net cash from (for) operating activities:
Stock-based compensation expense		54,000	61,000	275,000
Other assets		(1,805,000)	2,771,000	7,540,000
Change in other liabilities		2,257,000	(1,187,000)	(1,894,000)
Net cash from operating activities		17,021,000	22,506,000	29,602,000
Cash flows from investing activities
Net cash from investing activities		66,730,000	227,722,000	271,679,000
Cash flows from financing activities
Repurchase of preferred stock	(16,000,000)
Repurchase of common stock warrant		(7,465,000)	0	0
Stock option exercises, net of cashless exercises		227,000	55,000	0
Employee stock purchase plan		39,000	42,000	47,000
Dividend reinvestment plan		14,000	6,000	2,000
Payment of cash dividends on preferred stock		(496,000)	(1,620,000)	(525,000)
Payment of cash dividends to common shareholders		(774,000)	0	(85,000)
Net cash for financing activities		(24,120,000)	(238,054,000)	(258,818,000)
Net change in cash and cash equivalents		59,631,000	12,174,000	42,463,000
Cash and cash equivalents at beginning of period		76,372,000	64,198,000	21,735,000
Cash and cash equivalents at end of period		136,003,000	76,372,000	64,198,000
Parent Company [Member]
Cash flows from operating activities
Net income (loss)		12,535,000	37,485,000	(13,316,000)
Adjustments to reconcile net income (loss) to net cash from (for) operating activities:
Equity in undistributed net income (loss) of subsidiary		18,730,000	(32,145,000)	12,068,000
Stock-based compensation expense		54,000	61,000	275,000
Other assets		(3,006,000)	(3,619,000)	(124,000)
Change in other liabilities		1,073,000	(956,000)	963,000
Net cash from operating activities		29,386,000	826,000	(134,000)
Cash flows from investing activities
Net capital investment into subsidiaries		0	0	0
Net cash from investing activities		0	0	0
Cash flows from financing activities
Repurchase of preferred stock		(21,000,000)	0	0
Repurchase of common stock warrant		(7,465,000)	0	0
Stock option exercises, net of cashless exercises		227,000	55,000	0
Employee stock purchase plan		39,000	42,000	47,000
Dividend reinvestment plan		14,000	6,000	2,000
Payment of cash dividends on preferred stock		(496,000)	(1,620,000)	(525,000)
Payment of cash dividends to common shareholders		(774,000)	0	(85,000)
Net cash for financing activities		(29,455,000)	(1,517,000)	(561,000)
Net change in cash and cash equivalents		(69,000)	(691,000)	(695,000)
Cash and cash equivalents at beginning of period		542,000	1,233,000	1,928,000
Cash and cash equivalents at end of period		$ 473,000	$ 542,000	$ 1,233,000