Note 3 - Investment Securities - Amortized Cost, Unrealized Gains and Losses, and Estimated Fair Value of Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt securities available for sale, amortized cost	$ 2,718,210	$ 2,711,453
Debt securities available for sale, gross unrealized gains	13,443	3,597
Debt securities available for sale, gross unrealized losses	(29,413)	(60,380)
Debt securities available for sale	2,702,240	2,654,670
Debt securities held to maturity, amortized cost	923,190	984,609
Debt securities held to maturity, gross unrecognized gains	6,794	3,692
Debt securities held to maturity, gross unrecognized losses	(9,381)	(16,856)
Debt securities held to maturity, fair value	920,603	971,445
Debt securities, amortized cost	3,641,400	3,696,062
Debt securities, gross unrecognized gains	20,237	7,289
Debt securities, gross unrecognized losses	(38,794)	(77,236)
Debt securities, fair value	3,622,843	3,626,115
US Treasury Securities [Member]
Debt securities available for sale, amortized cost	109,000	139,572
Debt securities available for sale, gross unrealized gains	54	5
Debt securities available for sale, gross unrealized losses		(3)
Debt securities available for sale	109,054	139,574
US Government-sponsored Enterprises Debt Securities [Member]
Debt securities available for sale, amortized cost	167,234	167,228
Debt securities available for sale, gross unrealized gains	91	65
Debt securities available for sale, gross unrealized losses	(1,586)	(3,275)
Debt securities available for sale	165,739	164,018
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost	945,117	883,715
Debt securities available for sale, gross unrealized gains	3,599	595
Debt securities available for sale, gross unrealized losses	(18,693)	(30,439)
Debt securities available for sale	930,023	853,871
Debt securities held to maturity, amortized cost	427,387	447,332
Debt securities held to maturity, gross unrecognized gains	364	249
Debt securities held to maturity, gross unrecognized losses	(9,269)	(14,129)
Debt securities held to maturity, fair value	418,482	433,452
Non-agency Residential MBS [Member]
Debt securities available for sale, amortized cost	111	113
Debt securities available for sale, gross unrealized gains	1	1
Debt securities available for sale, gross unrealized losses
Debt securities available for sale	112	114
Debt securities held to maturity, amortized cost	3,124	3,387
Debt securities held to maturity, gross unrecognized gains	63	40
Debt securities held to maturity, gross unrecognized losses
Debt securities held to maturity, fair value	3,187	3,427
Agency Commercial MBS [Member]
Debt securities available for sale, amortized cost	1,860	1,869
Debt securities available for sale, gross unrealized gains
Debt securities available for sale, gross unrealized losses	(23)	(27)
Debt securities available for sale	1,837	1,842
US Government Agencies Debt Securities [Member]
Debt securities available for sale, amortized cost	1,091	1,128
Debt securities available for sale, gross unrealized gains
Debt securities available for sale, gross unrealized losses	(10)	(9)
Debt securities available for sale	1,081	1,119
US States and Political Subdivisions Debt Securities [Member]
Debt securities available for sale, amortized cost	169,947	180,220
Debt securities available for sale, gross unrealized gains	2,817	1,856
Debt securities available for sale, gross unrealized losses	(711)	(2,985)
Debt securities available for sale	172,053	179,091
Debt securities held to maturity, amortized cost	492,679	533,890
Debt securities held to maturity, gross unrecognized gains	6,367	3,403
Debt securities held to maturity, gross unrecognized losses	(112)	(2,727)
Debt securities held to maturity, fair value	498,934	534,566
Debt Security, Corporate, US [Member]
Debt securities available for sale, amortized cost	1,323,850	1,337,608
Debt securities available for sale, gross unrealized gains	6,881	1,075
Debt securities available for sale, gross unrealized losses	(8,390)	(23,642)
Debt securities available for sale	$ 1,322,341	$ 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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Amortized cost of 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 of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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

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