Note 3 - Investment Securities - Investment Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Due in 1 year or less, available for sale, amortized cost	$ 1,153
Due in 1 year or less, available for sale, fair value	1,156
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	6,613
After 1 year to 5 years, available for sale, fair value	6,624
After 1 year to 5 years, held to maturity, amortized cost	11,149
After 1 year to 5 years, held to maturity, fair value	11,116
After 5 years to 10 years, available for sale, amortized cost	79,701
After 5 years to 10 years, available for sale, fair value	76,993
After 5 years to 10 years, held to maturity, amortized cost	102,456
After 5 years to 10 years, held to maturity, fair value	100,304
After 10 years, available for sale, amortized cost	4,484
After 10 years, available for sale, fair value	4,550
After 10 years, held to maturity, amortized cost
After 10 years, held to maturity, fair value
Total, available for sale, amortized cost	475,587
Total, available for sale, fair value	464,430
Total, held to maturity, amortized cost	472,213	$ 432,499
Total, held to maturity, fair value	463,799	425,183
Collateralized Mortgage Obligations [Member]
No specific maturity date, available for sale, amortized cost	327,972
No specific maturity date, available for sale, fair value	320,241
No specific maturity date, held to maturity, amortized cost	215,567
No specific maturity date, held to maturity, fair value	211,911
Total, held to maturity, amortized cost	215,567	202,990
Total, held to maturity, fair value	211,911	$ 201,230
Collateralized Mortgage Backed Securities [Member]
No specific maturity date, available for sale, amortized cost	55,664
No specific maturity date, available for sale, fair value	54,866
No specific maturity date, held to maturity, amortized cost	143,041
No specific maturity date, held to maturity, fair value	$ 140,468