Note 4 - Investment Securities (Details Textual)	12 Months Ended
	Dec. 31, 2023 USD ($)		Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Debt Securities, Held-to-Maturity, Number of Securities	0
Debt Securities, Held-to-Maturity, Allowance for Credit Loss, Ending Balance	$ 0		$ 0
Debt Securities, Accrued Interest, after Allowance for Credit Loss	6,000,000		6,000,000
Debt Securities, Available-for-Sale, Realized Gain (Loss)	(44,000)	[1]	0	$ 0
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Ending Balance	$ 0		$ 0
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions	498		547
Debt Securities, Available-for-Sale, Unrealized Loss Position, Percentage	98.00%
Collateral Pledged [Member]
Debt Securities, Available-for-Sale, Restricted	$ 991,000,000,000		$ 1,090,000,000.00

[1] Outside of the scope of ASC 606.