Note 2 - Investment Securities - Amortized Cost, Unrealized Gains and Losses, and Estimated Fair Value of Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt securities available for sale, amortized cost	$ 4,269,948	$ 4,695,341
Debt securities available for sale, gross unrealized gains	1,360	1,625
Debt securities available for sale, gross unrealized losses	(271,507)	(365,223)
Debt securities available for sale	3,999,801	4,331,743
Held to maturity	878,397	915,914
Debt securities held to maturity, gross unrecognized gains	148	86
Debt securities held to maturity, gross unrecognized losses	(28,983)	(42,489)
Debt securities held to maturity, fair value	849,562	873,511
Debt securities, amortized cost	5,148,345	5,611,255
Debt securities, gross unrecognized gains	1,508	1,711
Debt securities, gross unrecognized losses	(300,490)	(407,712)
Total	4,849,363	5,205,254
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost	258,150	311,089
Debt securities available for sale, gross unrealized gains	6	4
Debt securities available for sale, gross unrealized losses	(18,702)	(25,045)
Debt securities available for sale	239,454	286,048
Held to maturity	78,565	104,852
Debt securities held to maturity, gross unrecognized gains	17	13
Debt securities held to maturity, gross unrecognized losses	(5,270)	(7,503)
Debt securities held to maturity, fair value	73,312	97,362
US Government Agencies Debt Securities [Member]
Debt securities available for sale, amortized cost	308,768	306,336
Debt securities available for sale, gross unrealized gains	2	3
Debt securities available for sale, gross unrealized losses	(13,851)	(15,486)
Debt securities available for sale	294,919	290,853
US States and Political Subdivisions Debt Securities [Member]
Debt securities available for sale, amortized cost	72,679	84,024
Debt securities available for sale, gross unrealized gains	42	59
Debt securities available for sale, gross unrealized losses	(1,438)	(2,079)
Debt securities available for sale	71,283	82,004
Held to maturity	71,182	89,208
Debt securities held to maturity, gross unrecognized gains	47	73
Debt securities held to maturity, gross unrecognized losses	(335)	(538)
Debt securities held to maturity, fair value	70,894	88,743
Corporate Debt Securities [Member]
Debt securities available for sale, amortized cost	2,129,103	2,406,566
Debt securities available for sale, gross unrealized gains	480	1,032
Debt securities available for sale, gross unrealized losses	(220,035)	(307,643)
Debt securities available for sale	1,909,548	2,099,955
Held to maturity	728,650	721,854
Debt securities held to maturity, gross unrecognized gains	84	0
Debt securities held to maturity, gross unrecognized losses	(23,378)	(34,448)
Debt securities held to maturity, fair value	705,356	687,406
Collateralized Debt Obligations [Member]
Debt securities available for sale, amortized cost	1,501,248	1,587,326
Debt securities available for sale, gross unrealized gains	830	527
Debt securities available for sale, gross unrealized losses	(17,481)	(14,970)
Debt securities available for sale	$ 1,484,597	$ 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 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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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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