Note 1 - Significant Accounting Policies (Details Textual)		1 Months Ended	3 Months Ended		6 Months Ended
	May 14, 2021 USD ($)	Apr. 30, 2020	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($) shares	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($) shares	May 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Aug. 08, 2020 USD ($)	Jul. 15, 2020 USD ($)
Number of Unconsolidated Business Trusts Formed to Issue Trust Preferred Securities			5		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			$ 28,900,000		$ 28,900,000				$ 24,000,000.0
Servicing Asset, Total			1,200,000,000		1,200,000,000				1,040,000,000.00
Property, Plant and Equipment, Net, Ending Balance			$ 58,250,000		$ 58,250,000				58,959,000
Revision of Prior Period, Adjustment [Member]
Property, Plant and Equipment, Net, Ending Balance								$ 2,700,000	2,800,000
Other Liabilities, Total								$ 2,700,000	$ 2,800,000
Hastings, Michigan Branch Office [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Accounts, Notes and Loans Receivable, Net	$ 9,700,000
Proceeds from Sale of Property Held-for-sale	1,500,000
Disposal Group, Including Discontinued Operation, Liabilities, Total	$ 14,600,000
Disposal Group, Including Discontinued Operation, Deposit Accounts, Premium of Book Balance	5.00%
Disposal Group, Including Discontinued Operation, Deposit Premium	$ 700,000
Hastings, Michigan Branch Office [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [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	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	2,000	3,000	2,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares			7,000	9,000	7,000	9,000
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			0		0						750
Financing Receivable, Principal and Interest Deferred Payments											$ 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			6	260	6	260
Financing Receivable, Principal and Interest Deferred Payments			$ 500,000	$ 23,800,000	$ 500,000	$ 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			1,900		1,900
Financing Receivable, Forgiveness Transactions			$ 487,000,000		$ 487,000,000
Amortization of Deferred Loan Origination Fees, Net					$ 3,500,000
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							$ 209,000,000
Financing Receivable, Forgiveness Transactions, Number of Loans			200		200
Financing Receivable, Forgiveness Transactions			$ 29,200,000		$ 29,200,000
Amortization of Deferred Loan Origination Fees, Net					$ 2,200,000