Note 3 - Investment Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) state security item	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Number of securities | security	541
Available-for-sale, unrealized loss positions, qualitative disclosure, number of positions | security	35
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	20
Other than temporary impairment losses, investments | $	$ 0	$ 0	$ 0
Sale of securities | $	30,055	1,938	71,092
Gain on sale | $	176		$ 67
Security Owned and Pledged as Collateral, Fair Value | $	$ 113,300	$ 100,900
Number of states holding investments | state	50
Number Of Charters Owning Municipal Securities | item	4
Municipal securities | General Obligation Bonds
Number of issuers | item	93	110
Other investments | $	$ 77,200	$ 86,400
Number of states holding investments | item	22	26
Municipal securities | General Obligation Bonds | Aggregate Fair Value Exceeding 5 Million [Member]
Number of states holding investments | item	6
Municipal securities | Revenue Bonds
Number of issuers | item	154	160
Other investments | $	$ 396,600	$ 371,900
Number of states holding investments | item	17	19
Revenue Bonds | Revenue Bonds | Minimum
Other investments | $	$ 5,000	$ 5,000
Revenue Bonds | Revenue Bonds | Aggregate Fair Value Exceeding 5 Million [Member]
Number of states holding investments | item	7	7
General Obligation Bonds | General Obligation Bonds | Minimum
Other investments | $	$ 5,000	$ 5,000
General Obligation Bonds | General Obligation Bonds | Aggregate Fair Value Exceeding 5 Million [Member]
Number of states holding investments | item		6