Note 24 - Mercantile Bank Corporation (Parent Company Only) Condensed Financial Statements - Condensed Balance Sheets (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 29, 2012
ASSETS
Other assets	$ 31,458,000	$ 28,976,000
Total assets	3,363,907,000	3,286,704,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	2,988,658,000	2,920,834,000
Subordinated debentures	46,199,000	45,517,000			$ 32,000,000
Shareholders’ equity	375,249,000	365,870,000	$ 340,811,000	$ 333,804,000
Total liabilities and shareholders’ equity	3,363,907,000	3,286,704,000
Parent Company [Member]
ASSETS
Cash and cash equivalents	9,653,000	9,704,000	$ 4,257,000	$ 4,940,000
Investment in bank subsidiary	394,561,000	383,941,000
Other assets	19,740,000	20,435,000
Total assets	423,954,000	414,080,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Liabilities	2,506,000	2,693,000
Subordinated debentures	46,199,000	45,517,000
Shareholders’ equity	375,249,000	365,870,000
Total liabilities and shareholders’ equity	$ 423,954,000	$ 414,080,000