Note 23 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Statements of Income (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income
Interest and dividends from subsidiaries	$ 39,564,000	$ 40,316,000	$ 39,820,000	$ 38,637,000	$ 37,195,000	$ 35,486,000	$ 34,319,000	$ 34,981,000	$ 158,337,000	$ 141,981,000	$ 125,543,000
Expenses
Interest expense									33,803,000	21,899,000	15,795,000
Income before income tax benefit and equity in undistributed net income of subsidiary									60,460,000	51,822,000	46,083,000
Federal income tax benefit									11,004,000	9,798,000	14,809,000
Net income	$ 13,317,000	$ 12,600,000	$ 11,715,000	$ 11,824,000	$ 11,574,000	$ 10,123,000	$ 9,446,000	$ 10,881,000	49,456,000	42,024,000	31,274,000
Comprehensive income									61,390,000	38,646,000	32,821,000
Parent Company [Member]
Income
Interest and dividends from subsidiaries									22,246,000	33,832,000	16,203,000
Total income									22,246,000	33,832,000	16,203,000
Expenses
Interest expense									3,153,000	2,999,000	2,496,000
Other operating expenses									4,804,000	4,424,000	3,651,000
Total expenses									7,957,000	7,423,000	6,147,000
Income before income tax benefit and equity in undistributed net income of subsidiary									14,289,000	26,409,000	10,056,000
Federal income tax benefit									(1,718,000)	(1,610,000)	(4,060,000)
Equity in undistributed net income of subsidiary									33,449,000	14,005,000	17,158,000
Net income									49,456,000	42,024,000	31,274,000
Comprehensive income									$ 61,390,000	$ 38,646,000	$ 32,821,000