Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Summary of Condensed Statements of Income (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income
Interest and dividends from subsidiaries	$ 15,373,000	$ 14,667,000	$ 13,992,000	$ 14,209,000	$ 14,666,000	$ 14,768,000	$ 14,930,000	$ 15,553,000	$ 58,242,000	$ 59,917,000	$ 71,069,000
Expenses
Interest expense									10,786,000	13,216,000	19,832,000
Income before income tax expense (benefit) and equity in undistributed net income (loss) of subsidiary									25,125,000	18,171,000	10,124,000
Federal income tax benefit									8,092,000	5,636,000	(27,361,000)
Net income									17,033,000	12,535,000	37,485,000
Preferred stock dividends and accretion									0	1,030,000	1,343,000
Net income attributable to common shares	5,163,000	3,453,000	4,016,000	4,400,000	3,049,000	2,616,000	3,288,000	2,552,000	17,033,000	11,505,000	36,142,000
Parent Company [Member]
Income
Interest and dividends from subsidiaries									5,516,000	32,532,000	4,974,000
Total income									5,516,000	32,532,000	4,974,000
Expenses
Interest expense									1,213,000	884,000	847,000
Other operating expenses									2,773,000	1,048,000	1,059,000
Total expenses									3,986,000	1,932,000	1,906,000
Income before income tax expense (benefit) and equity in undistributed net income (loss) of subsidiary									1,530,000	30,600,000	3,068,000
Federal income tax benefit									(1,042,000)	(665,000)	(2,272,000)
Equity in undistributed net income (loss) of subsidiary									14,461,000	(18,730,000)	32,145,000
Net income									17,033,000	12,535,000	37,485,000
Preferred stock dividends and accretion									0	1,030,000	1,343,000
Net income attributable to common shares									$ 17,033,000	$ 11,505,000	$ 36,142,000