Note 3 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Investment securities available for sale, amortized cost, 1 year or less	$ 331,756	$ 306,333
Investment securities available for sale, fair value, 1 year or less	329,956	309,257
Investment securities held to maturity, amortized cost, 1 year or less	14,686	15,836
Investment securities held to maturity, fair value, 1 year or less	14,634	15,941
Investment securities available for sale, amortized cost, over 1 to 5 years	557,420	707,062
Investment securities available for sale, fair value, over 1 to 5 years	523,563	738,057
Investment securities held to maturity, amortized cost, over 1 to 5 years	170,979	125,001
Investment securities held to maturity, fair value, over 1 to 5 years	165,738	127,539
Investment securities available for sale, amortized cost, over 5 to 10 years	2,823,019	2,320,559
Investment securities available for sale, fair value, over 5 to 10 years	2,482,869	2,347,242
Investment securities held to maturity, amortized cost, over 5 to 10 years	638,238	17,176
Investment securities held to maturity, fair value, over 5 to 10 years	593,977	17,615
Investment securities available for sale, amortized cost, over 10 years	750,409	834,395
Investment securities available for sale, fair value, over 10 years	743,254	832,573
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	4,462,604	4,168,349
Investment securities available for sale, fair value, subtotal	4,079,642	4,227,129
Investment securities held to maturity, amortized cost, subtotal	823,903	158,013
Investment securities held to maturity, fair value, subtotal	774,349	161,095
Investment securities available for sale, amortized cost	326,577	399,997
Investment securities available for sale, fair value	296,689	411,726
Investment securities held to maturity, amortized cost	112,371	148,390
Investment securities held to maturity, fair value	104,113	151,467
Investment securities available for sale, amortized cost	4,789,181	4,568,346
Investment securities available for sale, fair value	4,376,331	4,638,855
Investment securities held to maturity, amortized cost	936,274	306,403
Investment securities held to maturity, fair value	$ 878,462	$ 312,562