Note 1 - Significant Accounting Policies (Details Textual)		1 Months Ended	3 Months Ended				9 Months Ended		10 Months Ended
	Oct. 26, 2020 USD ($) ft²	Apr. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 shares	Nov. 04, 2020 USD ($)	Oct. 30, 2020 USD ($)	Jul. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
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
Financing Receivable, Held-for-Sale, Not Part of Disposal Group, after Valuation Allowance, Ending Balance			$ 27,800,000				$ 27,800,000						$ 5,000,000.0
Servicing Asset, Total			958,000,000				958,000,000						727,000,000
Goodwill, Impairment Loss			0	$ 0	$ 0
Financing Receivable, Advances under Paycheck Protection Program Liquidity Facility		$ 43,700,000	0				0
Loans and Leases Receivable, Allowance, Ending Balance			35,572,000				35,572,000						23,889,000
Retained Earnings (Accumulated Deficit), Ending Balance			124,451,000				124,451,000						$ 107,831,000
Accounting Standards Update 2016-02 [Member]
Operating Lease, Right-of-Use Asset														$ 1,300,000
Operating Lease, Liability, Total														$ 1,300,000
Accounting Standards Update 2016-13 [Member] | Scenario, Plan [Member]
Loans and Leases Receivable, Allowance, Ending Balance												$ (1,000,000.0)
Retained Earnings (Accumulated Deficit), Ending Balance												$ 800,000
SBA, CARES Act, Paycheck Protection Program [Member]
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums, Total			$ 15,000,000.0				$ 15,000,000.0
Core Deposits [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life (Year)							10 years
Subsequent Event [Member] | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member] | Lake Trust Credit Union [Member] | Branch Located in City of Hastings, Michigan [Member]
Area of Real Estate Property (Square Foot) | ft²	4,300
Disposal Group, Including Discontinued Operation, Deposits	$ 16,000,000
Disposal Group, Including Discontinued Operation, Consideration	$ 11,800,000
Disposal Group, Not Discontinued Operation, Deposit Premium	5.25%
Proceeds from Sale of Productive Assets, Total	$ 1,500,000
Subsequent Event [Member] | 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									$ 555,000,000
Proceeds from Collection of Loans Receivable									0
Subsequent Event [Member] | SBA, CARES Act, Paycheck Protection Program [Member] | Maximum [Member]
Financing Receivable, Forgiveness Transactions									$ 37,100,000
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											750
Financing Receivable With Principal and Interest Deferred Payments											$ 719,000,000
Commercial Portfolio Segment [Member] | Subsequent Event [Member]
Financing Receivable, Number of Principal and Interest Payment Deferrals										5
Financing Receivable With Principal and Interest Deferred Payments										$ 7,000,000.0
Retail Portfolio Segment [Member]
Financing Receivable, Payment Deferrals, Principal and Interest, Period (Day)		90 days
Financing Receivable, Number of Principal and Interest Payment Deferrals				260
Financing Receivable With Principal and Interest Deferred Payments				$ 23,800,000
Retail Portfolio Segment [Member] | Subsequent Event [Member]
Financing Receivable, Number of Principal and Interest Payment Deferrals										20
Financing Receivable With Principal and Interest Deferred Payments										$ 1,000,000.0
Restricted Stock [Member]
Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights (in shares) | shares			255,000			260,000	255,000	260,000
Share-based Payment Arrangement, Option [Member]
Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights (in shares) | shares			2,000			4,000	2,000	4,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares			9,000			7,000	9,000	7,000