Note 7 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment securities available for sale, amortized cost	$ 1,322,579	$ 1,267,731
Investment securities available for sale, gross unrealized gains	2,625	1,778
Investment securities available for sale, gross unrealized losses	15,351	27,000
Investment securities available for sale, Estimated fair value	1,309,853	1,242,509
US Treasury Securities [Member]
Investment securities available for sale, amortized cost	149,231	124,801
Investment securities available for sale, gross unrealized gains	20
Investment securities available for sale, gross unrealized losses	11	51
Investment securities available for sale, Estimated fair value	149,240	124,750
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	5,694	6,066
Investment securities available for sale, gross unrealized gains	3
Investment securities available for sale, gross unrealized losses	166	195
Investment securities available for sale, Estimated fair value	5,531	5,871
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale, amortized cost	400,000	400,000
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	6,726	11,637
Investment securities available for sale, Estimated fair value	393,274	388,363
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	701,761	670,874
Investment securities available for sale, gross unrealized gains	2,391	960
Investment securities available for sale, gross unrealized losses	8,191	15,089
Investment securities available for sale, Estimated fair value	695,961	656,745
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	905	1,005
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses	24	28
Investment securities available for sale, Estimated fair value	881	977
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	64,988	64,985
Investment securities available for sale, gross unrealized gains	211	818
Investment securities available for sale, gross unrealized losses	233
Investment securities available for sale, Estimated fair value	$ 64,966	$ 65,803

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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