Note 7 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment securities available for sale, amortized cost	$ 1,422,431	$ 1,267,731
Investment securities available for sale, gross unrealized gains	8,997	1,778
Investment securities available for sale, gross unrealized losses	3,990	27,000
Investment securities available for sale, Estimated fair value	1,427,438	1,242,509
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	92,477	6,066
Investment securities available for sale, gross unrealized gains	750	0
Investment securities available for sale, gross unrealized losses	273	195
Investment securities available for sale, Estimated fair value	92,954	5,871
US Treasury Securities [Member]
Investment securities available for sale, amortized cost		124,801
Investment securities available for sale, gross unrealized gains		0
Investment securities available for sale, gross unrealized losses		50
Investment securities available for sale, Estimated fair value		124,751
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale, amortized cost	350,000	400,000
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	2,301	11,638
Investment securities available for sale, Estimated fair value	347,699	388,362
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	880,406	670,874
Investment securities available for sale, gross unrealized gains	7,714	960
Investment securities available for sale, gross unrealized losses	1,385	15,089
Investment securities available for sale, Estimated fair value	886,735	656,745
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	683	1,005
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	18	28
Investment securities available for sale, Estimated fair value	665	977
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	98,865	64,985
Investment securities available for sale, gross unrealized gains	533	818
Investment securities available for sale, gross unrealized losses	13	0
Investment securities available for sale, Estimated fair value	$ 99,385	$ 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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