Note 2 - Securities - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Securities available for sale, amortized cost	$ 583,777	$ 577,406
Securities available-for-sale, unrealized gains	343	1,222
Securities available-for-sale, unrealized losses	(9,550)	(4,104)
Securities available-for-sale, fair value	574,570	574,524
US Government-sponsored Enterprises Debt Securities [Member]
Securities available for sale, amortized cost	392,833	214,852
Securities available-for-sale, unrealized gains	31	474
Securities available-for-sale, unrealized losses	(4,268)	(1,482)
Securities available-for-sale, fair value	388,596	213,844
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available for sale, amortized cost	148,068	163,571
Securities available-for-sale, unrealized gains	110	383
Securities available-for-sale, unrealized losses	(5,043)	(2,447)
Securities available-for-sale, fair value	143,135	161,507
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale, amortized cost	42,876	48,987
Securities available-for-sale, unrealized gains	202	365
Securities available-for-sale, unrealized losses	(239)	(163)
Securities available-for-sale, fair value	$ 42,839	49,189
US Treasury Securities [Member]
Securities available for sale, amortized cost		149,996
Securities available-for-sale, unrealized gains
Securities available-for-sale, unrealized losses		(12)
Securities available-for-sale, fair value		$ 149,984

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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