Note 4 - Securities - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities available-for-sale, amortized cost	$ 577,406	$ 571,936
Securities available-for-sale, unrealized gains	1,222	2,264
Securities available-for-sale, unrealized losses	(4,104)	(4,126)
Securities available-for-sale, fair value	574,524	570,074
US Treasury Securities [Member]
Securities available-for-sale, amortized cost	149,996	74,997
Securities available-for-sale, unrealized gains		1
Securities available-for-sale, unrealized losses	(12)
Securities available-for-sale, fair value	149,984	74,998
US Government-sponsored Enterprises Debt Securities [Member]
Securities available-for-sale, amortized cost	214,852	268,784
Securities available-for-sale, unrealized gains	474	800
Securities available-for-sale, unrealized losses	(1,482)	(1,494)
Securities available-for-sale, fair value	213,844	268,090
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available-for-sale, amortized cost	163,571	170,344
Securities available-for-sale, unrealized gains	383	735
Securities available-for-sale, unrealized losses	(2,447)	(2,236)
Securities available-for-sale, fair value	161,507	168,843
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, amortized cost	48,987	57,158
Securities available-for-sale, unrealized gains	365	682
Securities available-for-sale, unrealized losses	(163)	(396)
Securities available-for-sale, fair value	$ 49,189	57,444
Common Stock [Member]
Securities available-for-sale, amortized cost		653
Securities available-for-sale, unrealized gains		46
Securities available-for-sale, unrealized losses
Securities available-for-sale, fair value		$ 699

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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