Note 4 - Securities - Securities Available-for-sale and Held-to-maturity by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Securities held-to-maturity, amortized cost, due in one year or less	$ 1,353
Securities held-to-maturity, fair value, due in one year or less	1,353
Securities held-to-maturity, amortized cost, due after ten years	21,777
Securities held-to-maturity, fair value, due after ten years	19,928
Securities held-to-maturity, amortized cost	31,093	$ 30,886
Securities held-to-maturity, fair value	28,654	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	4,248
Securities available for sale, fair value, due after one year through five years	4,269
Securities available for sale, amortized cost, due after five years through ten years	125,569
Securities available for sale, fair value, due after five years through ten years	116,201
Securities available for sale, amortized cost, due after ten years	81,947
Securities available for sale, fair value, due after ten years	82,858
Securities available for sale, amortized cost	752,716	748,350
Securities available for sale, fair value, total other securities	728,623	738,354
Other Debt Obligations [Member]
Securities held-to-maturity, amortized cost	23,130	22,913
Securities held-to-maturity, fair value	21,281	21,889
Securities available for sale, fair value, total other securities	214,755	228,704
Collateralized Mortgage Backed Securities [Member]
Securities held-to-maturity, amortized cost	7,963	7,973
Securities held-to-maturity, fair value	7,373	7,810
Securities available for sale, amortized cost	529,525	513,985
Securities available for sale, fair value, total other securities	513,868	509,650
Total Other Securities [Member]
Securities available for sale, amortized cost	211,764
Securities available for sale, fair value, total other securities	203,328
Mutual Fund Debt Securities [Member]
Securities available for sale, amortized cost	11,427	11,575
Securities available for sale, fair value, total other securities	$ 11,427	$ 11,575