Note 5 - Investment Securities - Investment Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Due in 1 year or less, available for sale, amortized cost	$ 440
Due in 1 year or less, available for sale, fair value	441
Due in 1 year or less, held to maturity, amortized cost	757
Due in 1 year or less, held to maturity, fair value	766
After 1 year to 5 years, available for sale, amortized cost	50,361
After 1 year to 5 years, available for sale, fair value	49,908
After 1 year to 5 years, held to maturity, amortized cost	61,794
After 1 year to 5 years, held to maturity, fair value	64,794
After 5 years to 10 years, available for sale, amortized cost	55,693
After 5 years to 10 years, available for sale, fair value	52,670
After 5 years to 10 years, held to maturity, amortized cost	22,138
After 5 years to 10 years, held to maturity, fair value	23,615
After 10 years, available for sale, amortized cost	3,000
After 10 years, available for sale, fair value	2,631
After 10 years, held to maturity, amortized cost	0
After 10 years, held to maturity, fair value	0
Total, available for sale, amortized cost	440,456	$ 540,751
Total, available for sale, fair value	440,655	539,042
Total, held to maturity, amortized cost	688,939	644,842
Total, held to maturity, fair value	712,369	653,109
Collateralized Mortgage Obligations [Member]
No specific maturity date, available for sale, amortized cost	200,590
No specific maturity date, available for sale, fair value	203,802
No specific maturity date, held to maturity, amortized cost	320,408
No specific maturity date, held to maturity, fair value	334,654
Total, available for sale, amortized cost	200,590	329,492
Total, available for sale, fair value	203,802	331,438
Total, held to maturity, amortized cost	320,408	416,177
Total, held to maturity, fair value	334,654	$ 422,987
Collateralized Mortgage Backed Securities [Member]
No specific maturity date, available for sale, amortized cost	130,372
No specific maturity date, available for sale, fair value	131,203
No specific maturity date, held to maturity, amortized cost	283,842
No specific maturity date, held to maturity, fair value	$ 288,540