Note 3 - Investment Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) security item state	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Number of securities | security	668
Available-for-sale, unrealized loss positions, qualitative disclosure, number of positions | security	41
Aggregate losses of securities (as a percent)	0.10%
Available-for-sale, unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | security	5
Other than temporary impairment losses, investments | $	$ 0	$ 0	$ 0
Sale of securities | $	38,562	30,055	$ 1,938
Gain on sale | $	2,553	176
Security Owned and Pledged as Collateral, Fair Value | $	$ 212,800	$ 113,400
Number of states holding investments | state	50
Number Of Charters Owning Municipal Securities | item	4
Revenue Bonds
Number of issuers | item	2	1
Percentage threshold	5.00%	5.00%
Municipal securities | General Obligation Bonds
Number of issuers | item	117	93
Other investments | $	$ 116,700	$ 77,200
Number of states holding investments | item	21	22
Municipal securities | General Obligation Bonds | Minimum
Other investments | $	$ 5,000	$ 5,000
Municipal securities | General Obligation Bonds | Aggregate Fair Value Exceeding 5 Million [Member]
Number of states holding investments | item	8	6
Municipal securities | Revenue Bonds
Number of issuers | item	191	154
Other investments | $	$ 555,900	$ 396,600
Number of states holding investments | item	26	17
Municipal securities | Revenue Bonds | Minimum
Other investments | $	$ 5,000	$ 5,000
Municipal securities | Revenue Bonds | Aggregate Fair Value Exceeding 5 Million [Member]
Number of states holding investments | item	12	7