Note 4 - Securities - Securities Available-for-sale and Held-to-maturity by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Securities held-to-maturity, amortized cost, due in one year or less	$ 1,568
Securities held-to-maturity, fair value, due in one year or less	1,568
Securities held-to-maturity, amortized cost, due after ten years	21,639
Securities held-to-maturity, fair value, due after ten years	19,428
Securities held-to-maturity, amortized cost	31,165	$ 30,886
Securities held-to-maturity, fair value	28,217	29,699
Securities available for sale, amortized cost, due in one year or less	0
Securities available for sale, fair value, due in one year or less	0
Securities available for sale, amortized cost, due after one year through five years	4,219
Securities available for sale, fair value, due after one year through five years	4,215
Securities available for sale, amortized cost, due after five years through ten years	135,466
Securities available for sale, fair value, due after five years through ten years	125,245
Securities available for sale, amortized cost, due after ten years	91,538
Securities available for sale, fair value, due after ten years	92,048
Securities available for sale, amortized cost	790,241	748,350
Securities available for sale, fair value, total other securities	761,032	738,354
Other Debt Obligations [Member]
Securities held-to-maturity, amortized cost	23,207	22,913
Securities held-to-maturity, fair value	20,996	21,889
Securities available for sale, fair value, total other securities	232,913	228,704
Collateralized Mortgage Backed Securities [Member]
Securities held-to-maturity, amortized cost	7,958	7,973
Securities held-to-maturity, fair value	7,221	7,810
Securities available for sale, amortized cost	547,613	513,985
Securities available for sale, fair value, total other securities	528,119	509,650
Total Other Securities [Member]
Securities available for sale, amortized cost	231,223
Securities available for sale, fair value, total other securities	221,508
Mutual Fund Debt Securities [Member]
Securities available for sale, amortized cost	11,405	11,575
Securities available for sale, fair value, total other securities	$ 11,405	$ 11,575