Note 23 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Balance Sheets (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Other assets	$ 32,812,000	$ 31,458,000
Total assets	3,632,915,000	3,363,907,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	3,216,354,000	2,988,658,000
Subordinated debentures	46,881,000	46,199,000
Shareholders’ equity	416,561,000	375,249,000	$ 365,870,000	$ 340,811,000
Total liabilities and shareholders’ equity	3,632,915,000	3,363,907,000
Parent Company [Member]
ASSETS
Cash and cash equivalents	6,045,000	9,653,000	$ 9,704,000	$ 4,257,000
Investment in bank subsidiary	439,946,000	394,561,000
Other assets	18,032,000	19,740,000
Total assets	464,023,000	423,954,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	581,000	2,506,000
Subordinated debentures	46,881,000	46,199,000
Shareholders’ equity	416,561,000	375,249,000
Total liabilities and shareholders’ equity	$ 464,023,000	$ 423,954,000