Note 3 - Investment Securities - Amortized Cost, Unrealized Gains and Losses, and Estimated Fair Value of Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt securities available for sale, amortized cost	$ 2,434,038	$ 2,220,551
Debt securities available for sale, gross unrealized gains	3,428	5,424
Debt securities available for sale, gross unrealized losses	(74,272)	(34,268)
Debt securities available for sale	2,363,194	2,191,707
Debt securities held to maturity, amortized cost	1,076,456	1,158,864
Debt securities held to maturity, gross unrecognized gains	4,369	8,412
Debt securities held to maturity, gross unrecognized losses	(22,252)	(11,934)
Debt securities held to maturity, fair values	1,058,573	1,155,342
Debt securities, amortized cost	3,510,494	3,379,415
Debt securities, gross unrecognized gains	7,797	13,836
Debt securities, gross unrecognized losses	(96,524)	(46,202)
Debt securities, fair value	3,421,767	3,347,049
US Government-sponsored Enterprises Debt Securities [Member]
Debt securities available for sale, amortized cost	122,296	122,285
Debt securities available for sale, gross unrealized gains		1
Debt securities available for sale, gross unrealized losses	(4,924)	(2,967)
Debt securities available for sale	117,372	119,319
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost	894,339	787,679
Debt securities available for sale, gross unrealized gains	348	522
Debt securities available for sale, gross unrealized losses	(37,322)	(20,495)
Debt securities available for sale	857,365	767,706
Debt securities held to maturity, amortized cost	493,872	545,883
Debt securities held to maturity, gross unrecognized gains	256	606
Debt securities held to maturity, gross unrecognized losses	(18,857)	(9,850)
Debt securities held to maturity, fair values	475,271	536,639
Non-agency Residential MBS [Member]
Debt securities available for sale, amortized cost	127	153
Debt securities available for sale, gross unrealized gains	1	1
Debt securities available for sale, gross unrealized losses
Debt securities available for sale	128	154
Debt securities held to maturity, amortized cost	3,743	4,462
Debt securities held to maturity, gross unrecognized gains	74	70
Debt securities held to maturity, gross unrecognized losses
Debt securities held to maturity, fair values	3,817	4,532
Agency Commercial MBS [Member]
Debt securities available for sale, amortized cost	2,219	2,244
Debt securities available for sale, gross unrealized gains
Debt securities available for sale, gross unrealized losses	(41)	(25)
Debt securities available for sale	2,178	2,219
Debt securities held to maturity, amortized cost	1,886	9,041
Debt securities held to maturity, gross unrecognized gains
Debt securities held to maturity, gross unrecognized losses	(6)	(66)
Debt securities held to maturity, fair values	1,880	8,975
US Government Agencies Debt Securities [Member]
Debt securities available for sale, amortized cost	1,441	1,612
Debt securities available for sale, gross unrealized gains
Debt securities available for sale, gross unrealized losses	(8)	(22)
Debt securities available for sale	1,433	1,590
US States and Political Subdivisions Debt Securities [Member]
Debt securities available for sale, amortized cost	184,865	182,907
Debt securities available for sale, gross unrealized gains	2,466	3,796
Debt securities available for sale, gross unrealized losses	(3,640)	(1,482)
Debt securities available for sale	183,691	185,221
Debt securities held to maturity, amortized cost	576,955	599,478
Debt securities held to maturity, gross unrecognized gains	4,039	7,736
Debt securities held to maturity, gross unrecognized losses	(3,389)	(2,018)
Debt securities held to maturity, fair values	577,605	605,196
Debt Security, Corporate, US [Member]
Debt securities available for sale, amortized cost	1,228,751	1,123,671
Debt securities available for sale, gross unrealized gains	613	1,104
Debt securities available for sale, gross unrealized losses	(28,337)	(9,277)
Debt securities available for sale	$ 1,201,027	$ 1,115,498

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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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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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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Amount of investment in debt security measured at amortized cost (held-to-maturity) and 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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