Note 19 - Subordinated Debentures - Business Trusts (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Jan. 26, 2016
Preferred securities outstanding	$ 45,500,000	$ 44,800,000
Mercantile Bank Capital Trust I [Member]
Preferred securities outstanding	$ 21,000,000		$ 32,000,000.0
Mercantile Bank Capital Trust I [Member] | London Interbank Offered Rate (LIBOR) [Member]
Interest rate margin	2.18%
Firstbank Capital Trust I [Member]
Preferred securities outstanding	$ 10,000,000
Firstbank Capital Trust I [Member] | London Interbank Offered Rate (LIBOR) [Member]
Interest rate margin	1.99%
Firstbank Capital Trust II [Member]
Preferred securities outstanding	$ 10,000,000
Firstbank Capital Trust II [Member] | London Interbank Offered Rate (LIBOR) [Member]
Interest rate margin	1.27%
Firstbank Capital Trust III [Member]
Preferred securities outstanding	$ 7,500,000
Firstbank Capital Trust III [Member] | London Interbank Offered Rate (LIBOR) [Member]
Interest rate margin	1.35%
Firstbank Capital Trust IV [Member]
Preferred securities outstanding	$ 7,500,000
Firstbank Capital Trust IV [Member] | London Interbank Offered Rate (LIBOR) [Member]
Interest rate margin	1.35%