Note 2 - Investment Securities - Amortized Cost, Unrealized Gains and Losses, and Estimated Fair Value of Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt securities available for sale, amortized cost	$ 4,568,346	$ 3,900,751
Debt securities available for sale, gross unrealized gains	80,924	163,576
Debt securities available for sale, gross unrealized losses	(10,415)	(1,142)
Debt securities available for sale	4,638,855	4,063,185
Held to maturity	306,403	515,598
Debt securities held to maturity, gross unrecognized gains	6,196	14,121
Debt securities held to maturity, gross unrecognized losses	(37)	(32)
Debt securities held to maturity, fair value	312,562	529,687
Debt securities, amortized cost	4,874,749	4,416,349
Debt securities, gross unrecognized gains	87,120	177,697
Debt securities, gross unrecognized losses	(10,452)	(1,174)
Debt securities, fair value	4,951,417	4,592,872
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost	399,997	630,174
Debt securities available for sale, gross unrealized gains	11,766	22,779
Debt securities available for sale, gross unrealized losses	(37)	(1)
Debt securities available for sale	411,726	652,952
Held to maturity	147,632	240,332
Debt securities held to maturity, gross unrecognized gains	3,112	6,852
Debt securities held to maturity, gross unrecognized losses	(18)	(32)
Debt securities held to maturity, fair value	150,726	247,152
US Government Agencies Debt Securities [Member]
Debt securities available for sale, amortized cost	119	154
Debt securities available for sale, gross unrealized gains	0	0
Debt securities available for sale, gross unrealized losses	0	0
Debt securities available for sale	119	154
US States and Political Subdivisions Debt Securities [Member]
Debt securities available for sale, amortized cost	90,107	105,679
Debt securities available for sale, gross unrealized gains	3,842	5,332
Debt securities available for sale, gross unrealized losses	(29)	(1)
Debt securities available for sale	93,920	111,010
Held to maturity	158,013	273,922
Debt securities held to maturity, gross unrecognized gains	3,082	7,243
Debt securities held to maturity, gross unrecognized losses	0	0
Debt securities held to maturity, fair value	161,095	281,165
Debt Security, Corporate, US [Member]
Debt securities available for sale, amortized cost	2,692,792	1,986,995
Debt securities available for sale, gross unrealized gains	63,573	131,025
Debt securities available for sale, gross unrealized losses	(9,630)	(42)
Debt securities available for sale	2,746,735	2,117,978
Collateralized Debt Obligations [Member]
Debt securities available for sale, amortized cost	1,385,331	1,152,766
Debt securities available for sale, gross unrealized gains	1,743	4,433
Debt securities available for sale, gross unrealized losses	(719)	(1,098)
Debt securities available for sale	1,386,355	1,156,101
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Debt securities available for sale, amortized cost		24,983
Debt securities available for sale, gross unrealized gains		7
Debt securities available for sale, gross unrealized losses		0
Debt securities available for sale		24,990
Non-agency Residential MBS [Member]
Held to maturity	758	1,344
Debt securities held to maturity, gross unrecognized gains	2	26
Debt securities held to maturity, gross unrecognized losses	(19)	0
Debt securities held to maturity, fair value	$ 741	$ 1,370

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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