Note 21 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net income	$ 82,217	$ 61,063	$ 59,021
Adjustments to reconcile net income to net cash from operating activities:
Stock-based compensation expense	3,384	3,377	3,784
Stock grants to directors for retainer fee	350	359	344
Other assets	(30,189)	(437)	(12,655)
Accrued interest and other liabilities	14,252	32,524	(3,513)
Cash flows from investing activities
Net cash for investing activities	(395,932)	(554,999)	(495,507)
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises	0	36	50
Employee stock purchase plan	45	45	48
Dividend reinvestment plan	891	867	877
Net proceeds from subordinated notes issuance	0	14,645	73,635
Repurchases of common stock	0	0	(21,380)
Payment of cash dividends to common shareholders	(21,004)	(19,602)	(18,524)
Net cash (for) from financing activities	363,080	(443,251)	780,088
Net change in cash and cash equivalents	33,761	(878,388)	349,154
Cash and cash equivalents at beginning of period	96,772	975,160	626,006
Cash and cash equivalents at end of period	130,533	96,772	975,160
Parent Company [Member]
Cash flows from operating activities
Net income	82,217	61,063	59,021
Adjustments to reconcile net income to net cash from operating activities:
Equity in undistributed net income of subsidiary	(66,464)	(44,221)	(26,075)
Stock-based compensation expense	3,384	3,377	3,784
Stock grants to directors for retainer fee	350	359	344
Other assets	(128)	858	49
Accrued interest and other liabilities	1,045	1,636	(5)
Net cash from operating activities	20,404	23,072	37,118
Cash flows from investing activities
Net capital investment into subsidiaries	0	(15,000)	(70,000)
Net cash for investing activities	0	(15,000)	(70,000)
Cash flows from financing activities
Proceeds from stock option exercises, net of cashless exercises	0	36	50
Employee stock purchase plan	45	45	48
Dividend reinvestment plan	891	867	877
Net proceeds from subordinated notes issuance	0	14,645	73,635
Repurchases of common stock	0	0	(21,380)
Payment of cash dividends to common shareholders	(21,004)	(19,602)	(18,524)
Net cash (for) from financing activities	(20,068)	(4,009)	34,706
Net change in cash and cash equivalents	336	4,063	1,824
Cash and cash equivalents at beginning of period	18,633	14,570	12,746
Cash and cash equivalents at end of period	$ 18,969	$ 18,633	$ 14,570