Note 1 - Significant Accounting Policies (Details Textual)			1 Months Ended	3 Months Ended
	May 14, 2021 USD ($)	Oct. 21, 2020	Apr. 30, 2020	Mar. 31, 2021 USD ($) shares	Mar. 31, 2020 shares	Dec. 31, 2020 USD ($)	Aug. 08, 2020 USD ($)	Jul. 15, 2020 USD ($)	Jun. 30, 2020 USD ($)
Number of Unconsolidated Business Trusts Formed to Issue Trust Preferred Securities				5
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
Mortgages Held-for-sale, Fair Value Disclosure				$ 41,300,000		$ 24,000,000.0
Servicing Asset, Total				1,110,000,000		1,040,000,000.00
Disposal Group, Including Discontinued Operation, Liabilities, Total				17,280,000		$ 0
Hastings, Michigan Branch Office [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Deposit Accounts, Premium of Book Balance		5.00%
Disposal Group, Including Discontinued Operation, Accounts, Notes and Loans Receivable, Net				9,700,000
Disposal Group, Including Discontinued Operation, Property, Plant and Equipment				1,200,000
Disposal Group, Including Discontinued Operation, Liabilities, Total				$ 17,300,000
Hastings, Michigan Branch Office [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Forecast [Member]
Proceeds from Sale of Property Held-for-sale	$ 1,500,000
Disposal Group, Including Discontinued Operation, Deposit Premium	900,000
Hastings, Michigan Branch Office [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Forecast [Member] | Branch Facility [Member]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal	$ 300,000
Core Deposits [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life (Year)				10 years
Restricted Stock [Member]
Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights (in shares) | shares				262,000	256,000
Share-based Payment Arrangement, Option [Member]
Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights (in shares) | shares				3,000	4,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares				7,000	7,000
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				2				750
Financing Receivable, Principal and Interest Deferred Payments				$ 1,800,000				$ 719,000,000
Retail Portfolio Segment [Member]
Financing Receivable, Payment Deferrals, Principal and Interest, Period (Day)			90 days
Financing Receivable, Number of Principal and Interest Payment Deferrals				10					260
Financing Receivable, Principal and Interest Deferred Payments				$ 800,000					$ 23,800,000
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							$ 553,000,000
Financing Receivable, Forgiveness Transactions, Number of Loans				1,600
Financing Receivable, Forgiveness Transactions				$ 302,000,000
Amortization of Deferred Loan Origination Fees, Net				$ 2,400,000
SBA, CARES Act, Paycheck Protection Program, Round Two [Member]
Number of Loans Originated Under the Paycheck Protection Program				1,100
Financing Receivable, before Allowance for Credit Loss, Total				$ 203,000,000
Amortization of Deferred Loan Origination Fees, Net				$ 400,000