Note 3 - Investment Securities - Amortized Cost, Unrealized Gains and Losses, and Estimated Fair Value of Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt securities available for sale, amortized cost	$ 4,311,861	$ 4,695,341
Debt securities available for sale, gross unrealized gains	1,331	1,625
Debt securities available for sale, gross unrealized losses	(406,959)	(365,223)
Debt securities available for sale	3,906,233	4,331,743
Debt securities held to maturity, amortized cost	888,857	915,914
Debt securities held to maturity, gross unrecognized gains	13	86
Debt securities held to maturity, gross unrecognized losses	(70,475)	(42,489)
Debt securities held to maturity, fair value	818,395	873,511
Debt securities, amortized cost	5,200,718	5,611,255
Debt securities, gross unrecognized gains	1,344	1,711
Debt securities, gross unrecognized losses	(477,434)	(407,712)
Total	4,724,628	5,205,254
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost	269,883	311,089
Debt securities available for sale, gross unrealized gains	0	4
Debt securities available for sale, gross unrealized losses	(30,155)	(25,045)
Debt securities available for sale	239,728	286,048
Debt securities held to maturity, amortized cost	84,347	104,852
Debt securities held to maturity, gross unrecognized gains	9	13
Debt securities held to maturity, gross unrecognized losses	(8,116)	(7,503)
Debt securities held to maturity, fair value	76,240	97,362
US Government Agencies Debt Securities [Member]
Debt securities available for sale, amortized cost	308,160	306,336
Debt securities available for sale, gross unrealized gains	0	3
Debt securities available for sale, gross unrealized losses	(28,796)	(15,486)
Debt securities available for sale	279,364	290,853
US States and Political Subdivisions Debt Securities [Member]
Debt securities available for sale, amortized cost	73,614	84,024
Debt securities available for sale, gross unrealized gains	4	59
Debt securities available for sale, gross unrealized losses	(3,979)	(2,079)
Debt securities available for sale	69,639	82,004
Debt securities held to maturity, amortized cost	77,559	89,208
Debt securities held to maturity, gross unrecognized gains	4	73
Debt securities held to maturity, gross unrecognized losses	(1,957)	(538)
Debt securities held to maturity, fair value	75,606	88,743
Corporate Debt Securities [Member]
Debt securities available for sale, amortized cost	2,134,535	2,406,566
Debt securities available for sale, gross unrealized gains	625	1,032
Debt securities available for sale, gross unrealized losses	(320,736)	(307,643)
Debt securities available for sale	1,814,424	2,099,955
Debt securities held to maturity, amortized cost	726,951	721,854
Debt securities held to maturity, gross unrecognized gains	0	0
Debt securities held to maturity, gross unrecognized losses	(60,402)	(34,448)
Debt securities held to maturity, fair value	666,549	687,406
Collateralized Debt Obligations [Member]
Debt securities available for sale, amortized cost	1,525,669	1,587,326
Debt securities available for sale, gross unrealized gains	702	527
Debt securities available for sale, gross unrealized losses	(23,293)	(14,970)
Debt securities available for sale	$ 1,503,078	$ 1,572,883

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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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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Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity) and 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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