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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income
Interest and dividends from subsidiaries	$ 37,195,000	$ 35,486,000	$ 34,319,000	$ 34,981,000	$ 32,902,000	$ 33,034,000	$ 30,903,000	$ 28,704,000	$ 141,981,000	$ 125,543,000	$ 118,457,000
Expenses
Interest expense									21,899,000	15,795,000	12,590,000
Income before income tax benefit and equity in undistributed net income of subsidiary									51,822,000	46,083,000	46,887,000
Federal income tax benefit									9,798,000	14,809,000	14,974,000
Net income	$ 11,574,000	$ 10,123,000	$ 9,446,000	$ 10,881,000	$ 7,979,000	$ 8,337,000	$ 7,343,000	$ 7,615,000	42,024,000	31,274,000	31,913,000
Comprehensive income									38,646,000	32,821,000	25,069,000
Parent Company [Member]
Income
Interest and dividends from subsidiaries									33,832,000	16,203,000	32,521,000
Total income									33,832,000	16,203,000	32,521,000
Expenses
Interest expense									2,999,000	2,496,000	2,490,000
Other operating expenses									4,424,000	3,651,000	2,953,000
Total expenses									7,423,000	6,147,000	5,443,000
Income before income tax benefit and equity in undistributed net income of subsidiary									26,409,000	10,056,000	27,078,000
Federal income tax benefit									(1,610,000)	(4,060,000)	(836,000)
Equity in undistributed net income of subsidiary									14,005,000	17,158,000	3,999,000
Net income									42,024,000	31,274,000	31,913,000
Comprehensive income									$ 38,646,000	$ 32,821,000	$ 25,069,000