Note 20 - Subordinated Debentures (Details) - Business Trusts (USD $)	Dec. 31, 2014	Dec. 31, 2013
Subordinated Borrowing [Line Items]
Preferred securities outstanding	$ 52,400,000	$ 32,000,000
Mercantile Bank Capital Trust I [Member]
Subordinated Borrowing [Line Items]
Preferred securities outstanding	32,000,000
Firstbank Capital Trust I [Member]
Subordinated Borrowing [Line Items]
Preferred securities outstanding	10,000,000
Firstbank Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Preferred securities outstanding	10,000,000
Firstbank Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Preferred securities outstanding	7,500,000
Firstbank Capital Trust IV [Member]
Subordinated Borrowing [Line Items]
Preferred securities outstanding	$ 7,500,000