Note 3 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Investment securities available for sale, amortized cost, 1 year or less	$ 45,005	$ 251,578
Investment securities available for sale, fair value, 1 year or less	44,479	250,317
Investment securities held to maturity, amortized cost, 1 year or less	17,851	12,676
Investment securities held to maturity, fair value, 1 year or less	17,714	12,659
Investment securities available for sale, amortized cost, over 1 to 5 years	773,732	584,707
Investment securities available for sale, fair value, over 1 to 5 years	715,554	554,596
Investment securities held to maturity, amortized cost, over 1 to 5 years	273,548	161,653
Investment securities held to maturity, fair value, over 1 to 5 years	259,494	158,409
Investment securities available for sale, amortized cost, over 5 to 10 years	2,667,773	2,869,559
Investment securities available for sale, fair value, over 5 to 10 years	2,362,742	2,570,159
Investment securities held to maturity, amortized cost, over 5 to 10 years	513,111	636,733
Investment securities held to maturity, fair value, over 5 to 10 years	464,947	605,081
Investment securities available for sale, amortized cost, over 10 years	555,468	678,408
Investment securities available for sale, fair value, over 10 years	543,730	670,623
Investment securities held to maturity, amortized cost, over 10 years	0	0
Investment securities held to maturity, fair value, over 10 years	0	0
Investment securities available for sale, amortized cost, subtotal	4,041,978	4,384,252
Investment securities available for sale, fair value, subtotal	3,666,505	4,045,695
Investment securities held to maturity, amortized cost, subtotal	804,510	811,062
Investment securities held to maturity, fair value, subtotal	742,155	776,149
Investment securities available for sale, amortized cost	269,883	311,089
Investment securities available for sale, fair value	239,728	286,048
Investment securities held to maturity, amortized cost	84,347	104,852
Investment securities held to maturity, fair value	76,240	97,362
Investment securities available for sale, amortized cost	4,311,861	4,695,341
Investment securities available for sale, fair value	3,906,233	4,331,743
Investment securities held to maturity, amortized cost	888,857	915,914
Investment securities held to maturity, fair value	$ 818,395	$ 873,511