Note 3 - Loans and Allowance for Credit Losses on Loans (Details Textual)	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Loans and Leases Receivable, Deferred Expense (Income)	$ 11,000,000		$ 12,000,000
Pledged Financial Instruments, Not Separately Reported, Loans Receivable, for Federal Home Loan Bank Debt	2,000,000,000.0
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease), Total	$ (1,200,000)
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease), Percentage	(2.00%)
Financing Receivable, Nonaccrual, Interest Income	$ 0	$ 0
Financing Receivable, Deferred Payments	14,000,000		37,000,000
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 14,000,000		$ 37,000,000
Financing Receivable, Percent of Outstanding Loan Balance Deferred	0.45%		1.24%
Financing Receivable, Modifications, Number of Contracts	0	0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts	0	0
Mortgage Loans in Process of Foreclosure, Amount	$ 286,000		$ 147,000
Consumer Portfolio Segment [Member]
Financing Receivable, Overdraft Balance	492,000		536,000
Directors and Executive Officers [Member]
Loans and Leases Receivable, Related Parties, Ending Balance	$ 52,000,000		$ 43,000,000