Note 24 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Statements of Income (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income
Interest and dividends from subsidiaries	$ 29,715,000,000	$ 29,706,000,000	$ 30,147,000,000	$ 28,889,000,000	$ 28,575,000,000	$ 28,501,000,000	$ 27,663,000,000	$ 27,589,000,000	$ 118,457,000	$ 112,328,000	$ 89,118,000
Expenses
Interest expense									12,590,000	11,154,000	11,340,000
Income before federal income tax expense									46,887,000	38,831,000	25,196,000
Federal income tax expense									14,974,000	11,811,000	7,865,000
Net income									31,913,000	27,020,000	17,331,000
Comprehensive income									25,069,000	28,267,000	22,920,000
Parent Company [Member]
Income
Interest and dividends from subsidiaries									32,521,000	24,166,000	12,139,000
Total income									32,521,000	24,166,000	12,139,000
Expenses
Interest expense									2,490,000	2,569,000	2,145,000
Other operating expenses									2,953,000	2,276,000	3,552,000
Total expenses									5,443,000	4,845,000	5,697,000
Income before federal income tax expense									27,078,000	19,321,000	6,442,000
Federal income tax expense									(836,000)	(2,051,000)	(1,758,000)
Equity in undistributed net income of subsidiary									3,999,000	5,648,000	9,131,000
Net income									31,913,000	27,020,000	17,331,000
Comprehensive income									$ 25,069,000	$ 28,267,000	$ 22,920,000