Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands	1 Months Ended	9 Months Ended	12 Months Ended
	Apr. 30, 2020 USD ($)	Sep. 30, 2020	Dec. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) ft²	Feb. 26, 2021 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)	Jun. 01, 2014 USD ($)
Number of Unconsolidated Business Trusts Formed to Issue Trust Preferred Securities			5
Financing Receivable, Advances under Paycheck Protection Program Liquidity Facility	$ 43,700		$ 0
Loans and Leases Receivable, Deferred Income, Total			$ 4,400			$ 500
Interest Income on Commercial and Mortgage Loans Discontinued Period (Day)			90 days
Maximum Delinquency Period for Consumer and Credit Card Loans to be Charged Off (Day)			120 days
Repossessed Assets, Total			$ 700			$ 500
Accounting Standards Update 2016-02 [Member]
Operating Lease, Right-of-Use Asset							$ 1,300
Operating Lease, Liability, Total							$ 1,300
Banking Office in Hastings, Michigan [Member] | Forecast [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Area of Real Estate Property (Square Foot) | ft²				4,300
Disposal Group, Including Discontinued Operation, Deposits				$ 16,000
Core Deposits [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life (Year)		10 years
Commercial Portfolio Segment [Member]
Financing Receivable, Payment Deferrals, Interest-only Amendments, Period (Day)	90 days
Financing Receivable, Payment Deferrals, Principal and Interest, Period (Day)	90 days
Financing Receivable, Payment Deferrals, Interest Rate	0.00%
Financing Receivable, Number of Principal and Interest Payment Deferrals			19
Financing Receivable With Principal and Interest Deferred Payments			$ 8,000
Retail Portfolio Segment [Member]
Financing Receivable, Payment Deferrals, Principal and Interest, Period (Day)	90 days
Financing Receivable, Number of Principal and Interest Payment Deferrals			14
Financing Receivable With Principal and Interest Deferred Payments			$ 1,800
SBA, CARES Act, Paycheck Protection Program [Member]
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums, Total			$ 15,000
Number of Loans Originated Under the Paycheck Protection Program			2,200
Financing Receivable, before Allowance for Credit Loss, Total			$ 554,000
Financing Receivable, Forgiveness Transactions			$ 189,000
Financing Receivable, Forgiveness Transactions, Number of Loans			900
SBA, CARES Act, Paycheck Protection Program [Member] | Subsequent Event [Member]
Financing Receivable, Forgiveness Transactions					$ 266,000
Financing Receivable, Forgiveness Transactions, Number of Loans					1,400
SBA, CARES Act, Paycheck Protection Program, Second Round [Member] | Subsequent Event [Member]
Number of Loans Originated Under the Paycheck Protection Program					900
Financing Receivable, before Allowance for Credit Loss, Total					$ 181,000
Firstbank Corporation [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Total								$ 1,500,000
Business Combination Branches Acquired								46