Note 3 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Investment securities available for sale, amortized cost, 1 year or less	$ 122,825	$ 52,703
Investment securities available for sale, fair value, 1 year or less	121,872	52,357
Investment securities held to maturity, amortized cost, 1 year or less	12,626	15,117
Investment securities held to maturity, fair value, 1 year or less	12,568	15,095
Investment securities available for sale, amortized cost, over 1 to 5 years	869,312	756,658
Investment securities available for sale, fair value, over 1 to 5 years	817,287	721,179
Investment securities held to maturity, amortized cost, over 1 to 5 years	356,026	312,847
Investment securities held to maturity, fair value, over 1 to 5 years	346,397	307,557
Investment securities available for sale, amortized cost, over 5 to 10 years	1,479,251	1,701,189
Investment securities available for sale, fair value, over 5 to 10 years	1,282,243	1,502,214
Over 5 to 10 years	1,282,243	1,502,214
Investment securities held to maturity, amortized cost, over 5 to 10 years	424,440	471,868
Investment securities held to maturity, fair value, over 5 to 10 years	395,531	453,598
Investment securities available for sale, amortized cost, subtotal	2,471,388	2,510,550
Investment securities available for sale, fair value, subtotal	2,221,402	2,275,750
Investment securities held to maturity, amortized cost, subtotal	793,092	799,832
Investment securities held to maturity, fair value, subtotal	754,496	776,250
Debt securities available for sale, amortized cost	3,979,429	4,269,948
Debt securities available for sale	3,699,318	3,999,801
Debt securities held to maturity, amortized cost	860,869	878,397
Debt securities held to maturity, fair value	817,071	849,562
Collateralized Debt Obligations [Member]
Debt securities available for sale, amortized cost, without single date maturity	1,264,811	1,501,248
Debt securities available for sale, without single date maturity	1,255,110	1,484,597
Debt securities held to maturity, amortized cost, without single date maturity	0	0
Debt securities held to maturity, fair value, without single date maturity	0	0
Debt securities available for sale, amortized cost	1,264,811	1,501,248
Debt securities available for sale	1,255,110	1,484,597
Agency Residential MBS [Member]
Debt securities available for sale, amortized cost, without single date maturity	243,230	258,150
Debt securities available for sale, without single date maturity	222,806	239,454
Debt securities held to maturity, amortized cost, without single date maturity	67,777	78,565
Debt securities held to maturity, fair value, without single date maturity	62,575	73,312
Debt securities available for sale, amortized cost	243,230	258,150
Debt securities available for sale	222,806	239,454
Debt securities held to maturity, amortized cost	67,777	78,565
Debt securities held to maturity, fair value	$ 62,575	$ 73,312