Note 2 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Investment securities available for sale, amortized cost, 1 year or less	$ 52,703	$ 251,578
Investment securities available for sale, fair value, 1 year or less	52,357	250,317
Investment securities held to maturity, amortized cost, 1 year or less	15,117	12,676
Investment securities held to maturity, fair value, 1 year or less	15,095	12,659
Investment securities available for sale, amortized cost, over 1 to 5 years	756,658	578,886
Investment securities available for sale, fair value, over 1 to 5 years	721,179	548,787
Investment securities held to maturity, amortized cost, over 1 to 5 years	312,847	161,653
Investment securities held to maturity, fair value, over 1 to 5 years	307,557	158,409
Investment securities available for sale, amortized cost, over 5 to 10 years	1,701,189	1,966,462
Investment securities available for sale, fair value, over 5 to 10 years	1,502,214	1,673,708
Investment securities held to maturity, amortized cost, over 5 to 10 years	471,868	636,733
Investment securities held to maturity, fair value, over 5 to 10 years	453,598	605,081
Investment securities available for sale, amortized cost, subtotal	2,510,550	2,796,926
Investment securities available for sale, fair value, subtotal	2,275,750	2,472,812
Investment securities held to maturity, amortized cost, subtotal	799,832	811,062
Investment securities held to maturity, fair value, subtotal	776,250	776,149
Investment securities available for sale, amortized cost	4,269,948	4,695,341
Investment securities available for sale, fair value	3,999,801	4,331,743
Investment securities held to maturity, amortized cost	878,397	915,914
Investment securities held to maturity, fair value	849,562	873,511
Collateralized Debt Obligations [Member]
Investment securities available for sale, amortized cost	1,501,248	1,587,326
Investment securities available for sale, fair value	1,484,597	1,572,883
Investment securities held to maturity, amortized cost	0	0
Investment securities held to maturity, fair value	0	0
Investment securities available for sale, amortized cost	1,501,248	1,587,326
Investment securities available for sale, fair value	1,484,597	1,572,883
Agency Residential MBS [Member]
Investment securities available for sale, amortized cost	258,150	311,089
Investment securities available for sale, fair value	239,454	286,048
Investment securities held to maturity, amortized cost	78,565	104,852
Investment securities held to maturity, fair value	73,312	97,362
Investment securities available for sale, amortized cost	258,150	311,089
Investment securities available for sale, fair value	239,454	286,048
Investment securities held to maturity, amortized cost	78,565	104,852
Investment securities held to maturity, fair value	$ 73,312	$ 97,362