Note 5 - Investment Securities - Investment Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Due in 1 year or less, available for sale, amortized cost	$ 3,439
Due in 1 year or less, available for sale, fair value	3,449
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	2,990
After 1 year to 5 years, available for sale, fair value	3,009
After 1 year to 5 years, held to maturity, amortized cost	12,916
After 1 year to 5 years, held to maturity, fair value	12,923
After 5 years to 10 years, available for sale, amortized cost	65,145
After 5 years to 10 years, available for sale, fair value	61,965
After 5 years to 10 years, held to maturity, amortized cost	91,194
After 5 years to 10 years, held to maturity, fair value	88,982
After 10 years, available for sale, amortized cost	4,486
After 10 years, available for sale, fair value	4,532
After 10 years, held to maturity, amortized cost
After 10 years, held to maturity, fair value
Total, available for sale, amortized cost	291,440	$ 326,740
Total, available for sale, fair value	287,694	321,014
Total, held to maturity, amortized cost	742,435	761,563
Total, held to maturity, fair value	736,735	747,323
Collateralized Mortgage Obligations [Member]
No specific maturity date, available for sale, amortized cost	177,000
No specific maturity date, available for sale, fair value	176,508
No specific maturity date, held to maturity, amortized cost	488,402
No specific maturity date, held to maturity, fair value	487,046
Total, available for sale, amortized cost	177,000	197,812
Total, available for sale, fair value	176,508	196,259
Total, held to maturity, amortized cost	488,402	500,690
Total, held to maturity, fair value	487,046	$ 495,467
Collateralized Mortgage Backed Securities [Member]
No specific maturity date, available for sale, amortized cost	38,380
No specific maturity date, available for sale, fair value	38,231
No specific maturity date, held to maturity, amortized cost	149,923
No specific maturity date, held to maturity, fair value	$ 147,784