Note 3 - Investment Securities - Amortized Cost, Unrealized Gains and Losses, and Estimated Fair Value of Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt securities available for sale, amortized cost	$ 3,575,297	$ 3,041,860
Debt securities available for sale, gross unrealized gains	134,488	43,857
Debt securities available for sale, gross unrealized losses	(1,415)	(6,871)
Debt securities available for sale	3,708,370	3,078,846
Held to maturity	638,297	738,072
Debt securities held to maturity, gross unrecognized gains	18,138	8,460
Debt securities held to maturity, gross unrecognized losses	(45)	(2,236)
Debt securities held to maturity, fair value	656,390	744,296
Debt securities, amortized cost	4,213,594	3,779,932
Debt securities, gross unrecognized gains	152,626	52,317
Debt securities, gross unrecognized losses	(1,460)	(9,107)
Debt securities, fair value	4,364,760	3,823,142
Collateralized Loan Obligations [Member]
Debt securities available for sale, gross unrealized gains
Debt securities available for sale, gross unrealized losses
Debt securities available for sale
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost	811,331	934,592
Debt securities available for sale, gross unrealized gains	28,755	10,996
Debt securities available for sale, gross unrealized losses	(1)	(5,838)
Debt securities available for sale	840,085	939,750
Held to maturity	304,439	353,937
Debt securities held to maturity, gross unrecognized gains	9,364	766
Debt securities held to maturity, gross unrecognized losses	(45)	(2,235)
Debt securities held to maturity, fair value	313,758	352,468
US Treasury Securities [Member]
Debt securities available for sale, amortized cost		19,999
Debt securities available for sale, gross unrealized gains		1
Debt securities available for sale, gross unrealized losses		0
Debt securities available for sale		20,000
Agency Commercial MBS [Member]
Debt securities available for sale, amortized cost	3,638	3,711
Debt securities available for sale, gross unrealized gains	0	0
Debt securities available for sale, gross unrealized losses	(9)	(3)
Debt securities available for sale	3,629	3,708
US Government-sponsored Enterprises Debt Securities [Member]
Debt securities available for sale, amortized cost		111,251
Debt securities available for sale, gross unrealized gains		14
Debt securities available for sale, gross unrealized losses		(98)
Debt securities available for sale		111,167
US Government Agencies Debt Securities [Member]
Debt securities available for sale, amortized cost	171	553
Debt securities available for sale, gross unrealized gains	0	0
Debt securities available for sale, gross unrealized losses	0	(9)
Debt securities available for sale	171	544
US States and Political Subdivisions Debt Securities [Member]
Debt securities available for sale, amortized cost	147,260	159,527
Debt securities available for sale, gross unrealized gains	4,932	3,656
Debt securities available for sale, gross unrealized losses	(38)	(44)
Debt securities available for sale	152,154	163,139
Held to maturity	331,746	381,781
Debt securities held to maturity, gross unrecognized gains	8,760	7,672
Debt securities held to maturity, gross unrecognized losses	0	(1)
Debt securities held to maturity, fair value	340,506	389,452
Debt Security, Corporate, US [Member]
Debt securities available for sale, amortized cost	2,170,503	1,805,479
Debt securities available for sale, gross unrealized gains	100,018	29,183
Debt securities available for sale, gross unrealized losses	(1,110)	(879)
Debt securities available for sale	2,269,411	1,833,783
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Debt securities available for sale, amortized cost	299,742
Debt securities available for sale, gross unrealized gains	45
Debt securities available for sale, gross unrealized losses	(205)
Debt securities available for sale	299,582
Collateralized Debt Obligations [Member]
Debt securities available for sale, amortized cost	142,652	6,748
Debt securities available for sale, gross unrealized gains	738	7
Debt securities available for sale, gross unrealized losses	(52)	0
Debt securities available for sale	143,338	6,755
Non-agency Residential MBS [Member]
Held to maturity	2,112	2,354
Debt securities held to maturity, gross unrecognized gains	14	22
Debt securities held to maturity, gross unrecognized losses	0	0
Debt securities held to maturity, fair value	$ 2,126	$ 2,376

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 unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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