Note 8 - Investment Securities - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Held to maturity investments for less than one year	$ 23,022	$ 19,793
Held to maturity investments for one to five years	3,794	5,085
Held to maturity, total	26,816	24,878
Commercial Paper Original Maturities of 91 Days or More [Member]
Held to maturity investments for less than one year	13,389	7,490
Held to maturity investments for one to five years	0	0
Held to maturity, total	13,389	7,490
Corporate Notes And Bonds [Member]
Held to maturity investments for less than one year	9,633	12,303
Held to maturity investments for one to five years	3,794	5,085
Held to maturity, total	$ 13,427	$ 17,388