Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jan. 01, 2022 USD ($)	May 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Aug. 08, 2020 USD ($)	Jun. 01, 2014 USD ($)
Number of Unconsolidated Business Trusts Formed to Issue Trust Preferred Securities	5
Financing Receivable, before Allowance for Credit Loss, Total	$ 4,303,758	$ 3,916,619
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Ending Balance	0
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss	2,600	2,500
Financing Receivable, Unamortized Loan Cost (Fee)	$ 2,400	1,000
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
Financing Receivable, Accrued Interest, after Allowance for Credit Loss	$ 16,900	12,800
Repossessed Assets	200	0
Financing Receivable, Allowance for Credit Loss	49,914	42,246	$ 35,363			$ 37,967
Retained Earnings (Accumulated Deficit)	277,526	216,313
Other Expense [Member]
Income (Loss) from Affordable Housing Projects, Equity Method Investments		(300)	(200)
Federal Income Tax Expense [Member]
Amortization Method Qualified Affordable Housing Project Investments, Amortization	500
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Financing Receivable, Allowance for Credit Loss			$ (400)	$ (400)
Retained Earnings (Accumulated Deficit)				300
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Other Assets [Member]
Deferred Income Tax Assets, Net				$ 100
Not Designated as Hedging Instrument [Member] | Mortgage Loan Derivative [Member]
Derivative Liability	100
Derivative Asset		100
Commercial Loan [Member]
Loan Participation, Amount	46,700	$ 33,500
SBA, CARES Act, Paycheck Protection Program [Member]
Number of Loans Originated Under the Paycheck Protection Program							2,200
Financing Receivable, before Allowance for Credit Loss, Total							$ 554,000
Financing Receivable, Not Yet Forgiven	100
SBA, CARES Act, Paycheck Protection Program, Round Two [Member]
Number of Loans Originated Under the Paycheck Protection Program					1,200
Financing Receivable, before Allowance for Credit Loss, Total					$ 209,000
Financing Receivable, Not Yet Forgiven	$ 200
Financing Receivable, Not Yet Forgiven, Number of Loans	5
Firstbank Corporation [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Total								$ 1,500,000
Business Combination Branches Acquired								46