Note 2 - Securities (Details Textual)	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	64
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 119,073,000	$ 117,573,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	899,000	1,254,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held, Ending Balance	0
Proceeds from Sale of Available-for-sale Securities, Total	0	0	$ 0
Debt Securities, Available-for-sale, Amortized Cost, Total	380,403,000	329,979,000
Debt Securities, Available-for-sale, Total	387,347,000	334,655,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	109,000,000	96,500,000
Debt Securities, Available-for-sale, Total	116,000,000	99,400,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	4,100,000	5,900,000
Debt Securities, Available-for-sale, Total	4,200,000	5,900,000
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Restricted	$ 118,000,000	$ 103,000,000