Note 18 - Variable Interest Entities - Business Trusts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Jan. 26, 2016
Preferred securities outstanding	$ 47.6	$ 46.9
Mercantile Bank Capital Trust I [Member]
Preferred securities outstanding	$ 21.0		$ 32.0
Mercantile Bank Capital Trust I [Member] | Secured Overnight Financing Rate (SOFR) [Member]
Interest rate margin	2.18%
Firstbank Capital Trust I [Member]
Preferred securities outstanding	$ 10.0
Firstbank Capital Trust I [Member] | Secured Overnight Financing Rate (SOFR) [Member]
Interest rate margin	1.99%
Firstbank Capital Trust II [Member]
Preferred securities outstanding	$ 10.0
Firstbank Capital Trust II [Member] | Secured Overnight Financing Rate (SOFR) [Member]
Interest rate margin	1.27%
Firstbank Capital Trust III [Member]
Preferred securities outstanding	$ 7.5
Firstbank Capital Trust III [Member] | Secured Overnight Financing Rate (SOFR) [Member]
Interest rate margin	1.35%
Firstbank Capital Trust IV [Member]
Preferred securities outstanding	$ 7.5
Firstbank Capital Trust IV [Member] | Secured Overnight Financing Rate (SOFR) [Member]
Interest rate margin	1.35%