Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Summary of Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
ASSETS
Other assets	$ 5,911,000	$ 15,310,000
Total assets	1,426,966,000	1,422,926,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	1,273,641,000	1,276,336,000
Subordinated debentures	32,990,000	32,990,000
Shareholders' equity	153,325,000	146,590,000	164,999,000	125,936,000
Total liabilities and shareholders' equity	1,426,966,000	1,422,926,000
Parent Company [Member]
ASSETS
Cash and cash equivalents	2,506,000	473,000	542,000	1,233,000
Investment in bank subsidiary	179,706,000	173,019,000
Other assets	4,740,000	7,433,000
Total assets	186,952,000	180,925,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	637,000	1,345,000
Subordinated debentures	32,990,000	32,990,000
Shareholders' equity	153,325,000	146,590,000
Total liabilities and shareholders' equity	$ 186,952,000	$ 180,925,000