Note 3 - Investment Securities - Amortized Cost, Unrealized Gains and Losses, and Estimated Fair Value of Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt securities available for sale, amortized cost	$ 2,757,267	$ 2,711,453
Debt securities available for sale, gross unrealized gains	29,361	3,597
Debt securities available for sale, gross unrealized losses	(10,729)	(60,380)
Debt securities available for sale	2,775,899	2,654,670
Debt securities held to maturity, amortized cost	867,989	984,609
Debt securities held to maturity, gross unrecognized gains	8,867	3,692
Debt securities held to maturity, gross unrecognized losses	(3,880)	(16,856)
Debt securities held to maturity, fair value	872,976	971,445
Debt securities, amortized cost	3,625,256	3,696,062
Debt securities, gross unrecognized gains	38,228	7,289
Debt securities, gross unrecognized losses	(14,609)	(77,236)
Debt securities, fair value	3,648,875	3,626,115
US Treasury Securities [Member]
Debt securities available for sale, amortized cost	44,473	139,572
Debt securities available for sale, gross unrealized gains	100	5
Debt securities available for sale, gross unrealized losses	0	(3)
Debt securities available for sale	44,573	139,574
US Government-sponsored Enterprises Debt Securities [Member]
Debt securities available for sale, amortized cost	167,239	167,228
Debt securities available for sale, gross unrealized gains	69	65
Debt securities available for sale, gross unrealized losses	(170)	(3,275)
Debt securities available for sale	167,138	164,018
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost	987,024	883,715
Debt securities available for sale, gross unrealized gains	8,236	595
Debt securities available for sale, gross unrealized losses	(8,314)	(30,439)
Debt securities available for sale	986,946	853,871
Debt securities held to maturity, amortized cost	402,961	447,332
Debt securities held to maturity, gross unrecognized gains	824	249
Debt securities held to maturity, gross unrecognized losses	(3,861)	(14,129)
Debt securities held to maturity, fair value	399,924	433,452
Non-agency Residential MBS [Member]
Debt securities available for sale, amortized cost	109	113
Debt securities available for sale, gross unrealized gains	3	1
Debt securities available for sale, gross unrealized losses	0	0
Debt securities available for sale	112	114
Debt securities held to maturity, amortized cost	2,610	3,387
Debt securities held to maturity, gross unrecognized gains	96	40
Debt securities held to maturity, gross unrecognized losses	0	0
Debt securities held to maturity, fair value	2,706	3,427
Agency Commercial MBS [Member]
Debt securities available for sale, amortized cost	5,635	1,869
Debt securities available for sale, gross unrealized gains	5	0
Debt securities available for sale, gross unrealized losses	(19)	(27)
Debt securities available for sale	5,621	1,842
US Government Agencies Debt Securities [Member]
Debt securities available for sale, amortized cost	815	1,128
Debt securities available for sale, gross unrealized gains	0	0
Debt securities available for sale, gross unrealized losses	(7)	(9)
Debt securities available for sale	808	1,119
US States and Political Subdivisions Debt Securities [Member]
Debt securities available for sale, amortized cost	172,268	180,220
Debt securities available for sale, gross unrealized gains	3,486	1,856
Debt securities available for sale, gross unrealized losses	(100)	(2,985)
Debt securities available for sale	175,654	179,091
Debt securities held to maturity, amortized cost	462,418	533,890
Debt securities held to maturity, gross unrecognized gains	7,947	3,403
Debt securities held to maturity, gross unrecognized losses	(19)	(2,727)
Debt securities held to maturity, fair value	470,346	534,566
Debt Security, Corporate, US [Member]
Debt securities available for sale, amortized cost	1,379,704	1,337,608
Debt securities available for sale, gross unrealized gains	17,462	1,075
Debt securities available for sale, gross unrealized losses	(2,119)	(23,642)
Debt securities available for sale	$ 1,395,047	$ 1,315,041

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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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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