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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income
Interest and dividends from subsidiaries	$ 32,902,000	$ 33,034,000	$ 30,903,000	$ 28,704,000	$ 29,715,000	$ 29,706,000	$ 30,147,000	$ 28,889,000	$ 125,543,000	$ 118,457,000	$ 112,328,000
Expenses
Interest expense									15,795,000	12,590,000	11,154,000
Income before federal income tax expense									46,083,000	46,887,000	38,831,000
Federal income tax expense									14,809,000	14,974,000	11,811,000
Net income	$ 7,979,000	$ 8,337,000	$ 7,343,000	$ 7,615,000	$ 8,085,000	$ 7,845,000	$ 7,434,000	$ 8,549,000	31,274,000	31,913,000	27,020,000
Comprehensive income									32,821,000	25,069,000	28,267,000
Parent Company [Member]
Income
Interest and dividends from subsidiaries									16,203,000	32,521,000	24,166,000
Total income									16,203,000	32,521,000	24,166,000
Expenses
Interest expense									2,496,000	2,490,000	2,569,000
Other operating expenses									3,651,000	2,953,000	2,276,000
Total expenses									6,147,000	5,443,000	4,845,000
Income before federal income tax expense									10,056,000	27,078,000	19,321,000
Federal income tax expense									(4,060,000)	(836,000)	(2,051,000)
Equity in undistributed net income of subsidiary									17,158,000	3,999,000	5,648,000
Net income									31,274,000	31,913,000	27,020,000
Comprehensive income									$ 32,821,000	$ 25,069,000	$ 28,267,000