Note 8 - Investment Securities - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Held to maturity investments for less than one year	$ 20,044	$ 19,793
Held to maturity investments for one to five years	0	5,085
Held to maturity, total	20,044	24,878
Commercial Paper Original Maturities of 91 Days or More [Member]
Held to maturity investments for less than one year	11,142	7,490
Held to maturity investments for one to five years	0	0
Held to maturity, total	11,142	7,490
Corporate Notes And Bonds [Member]
Held to maturity investments for less than one year	8,902	12,303
Held to maturity investments for one to five years	0	5,085
Held to maturity, total	$ 8,902	$ 17,388