Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements (Details 1) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income
Interest and dividends from subsidiaries	$ 14,666,000	$ 14,768,000	$ 14,930,000	$ 15,553,000	$ 16,406,000	$ 17,044,000	$ 18,460,000	$ 19,159,000	$ 20,524,000	$ 21,734,000	$ 22,696,000	$ 23,189,000	$ 59,917,000	$ 71,069,000	$ 88,143,000
Expenses
Interest expense													13,216,000	19,832,000	31,794,000
Income (loss) before income tax expense (benefit) and equity in undistributed net income (loss) of subsidiary													18,171,000	10,124,000	(13,363,000)
Federal income tax expense (benefit)					27,400,000								5,636,000	(27,361,000)	(47,000)
Net income (loss)													12,535,000	37,485,000	(13,316,000)
Preferred stock dividends and accretion													1,030,000	1,343,000	1,295,000
Net income (loss) attributable to common shares	3,049,000	2,616,000	3,288,000	2,552,000	29,991,000	2,682,000	2,381,000	1,088,000	(5,282,000)	(5,682,000)	(684,000)	(2,963,000)	11,505,000	36,142,000	(14,611,000)
Parent Company [Member]
Income
Interest and dividends from subsidiaries													32,532,000	4,974,000	1,104,000
Total income													32,532,000	4,974,000	1,104,000
Expenses
Interest expense													884,000	847,000	848,000
Other operating expenses													1,048,000	1,059,000	1,551,000
Total expenses													1,932,000	1,906,000	2,399,000
Income (loss) before income tax expense (benefit) and equity in undistributed net income (loss) of subsidiary													30,600,000	3,068,000	(1,295,000)
Federal income tax expense (benefit)													(665,000)	(2,272,000)	(47,000)
Equity in undistributed net income (loss) of subsidiary													(18,730,000)	32,145,000	(12,068,000)
Net income (loss)													12,535,000	37,485,000	(13,316,000)
Preferred stock dividends and accretion													1,030,000	1,343,000	1,295,000
Net income (loss) attributable to common shares													$ 11,505,000	$ 36,142,000	$ (14,611,000)