Note 3 - Investment Securities - Investment Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Due in 1 year or less, available for sale, amortized cost	$ 3,790
Due in 1 year or less, available for sale, fair value	3,795
Due in 1 year or less, held to maturity, amortized cost
Due in 1 year or less, held to maturity, fair value
After 1 year to 5 years, available for sale, amortized cost	39,653
After 1 year to 5 years, available for sale, fair value	39,435
After 1 year to 5 years, held to maturity, amortized cost	20,048
After 1 year to 5 years, held to maturity, fair value	20,073
After 5 years to 10 years, available for sale, amortized cost	65,863
After 5 years to 10 years, available for sale, fair value	62,617
After 5 years to 10 years, held to maturity, amortized cost	74,865
After 5 years to 10 years, held to maturity, fair value	75,028
After 10 years, available for sale, amortized cost	3,000
After 10 years, available for sale, fair value	2,820
After 10 years, held to maturity, amortized cost
After 10 years, held to maturity, fair value
Total, available for sale, amortized cost	540,751	$ 326,740
Total, available for sale, fair value	539,042	321,014
Total, held to maturity, amortized cost	644,842	761,563
Total, held to maturity, fair value	653,109	747,323
Collateralized Mortgage Obligations [Member]
No specific maturity date, available for sale, amortized cost	329,492
No specific maturity date, available for sale, fair value	331,438
No specific maturity date, held to maturity, amortized cost	416,177
No specific maturity date, held to maturity, fair value	422,987
Total, available for sale, amortized cost	329,492	197,812
Total, available for sale, fair value	331,438	196,259
Total, held to maturity, amortized cost	416,177	500,690
Total, held to maturity, fair value	422,987	$ 495,467
Collateralized Mortgage Backed Securities [Member]
No specific maturity date, available for sale, amortized cost	98,953
No specific maturity date, available for sale, fair value	98,937
No specific maturity date, held to maturity, amortized cost	133,752
No specific maturity date, held to maturity, fair value	$ 135,021