Note 2 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Investment securities available for sale, amortized cost, 1 year or less	$ 306,333	$ 212,140
Investment securities available for sale, fair value, 1 year or less	309,257	213,715
Investment securities held to maturity, amortized cost, 1 year or less	15,836	54,526
Investment securities held to maturity, fair value, 1 year or less	15,941	54,927
Investment securities available for sale, amortized cost, over 1 to 5 years	707,062	922,170
Investment securities available for sale, fair value, over 1 to 5 years	738,057	974,438
Investment securities held to maturity, amortized cost, over 1 to 5 years	125,001	129,786
Investment securities held to maturity, fair value, over 1 to 5 years	127,539	133,195
Investment securities available for sale, amortized cost, over 5 to 10 years	2,320,559	1,767,747
Investment securities available for sale, fair value, over 5 to 10 years	2,347,242	1,851,184
Investment securities held to maturity, amortized cost, over 5 to 10 years	17,176	89,610
Investment securities held to maturity, fair value, over 5 to 10 years	17,615	93,043
Investment securities available for sale, amortized cost, over 10 years	834,395	368,520
Investment securities available for sale, fair value, over 10 years	832,573	370,896
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	4,168,349	3,270,577
Investment securities available for sale, fair value, subtotal	4,227,129	3,410,233
Investment securities held to maturity, amortized cost, subtotal	158,013	273,922
Investment securities held to maturity, fair value, subtotal	161,095	281,165
Investment securities available for sale, amortized cost	399,997	630,174
Investment securities available for sale, fair value	411,726	652,952
Investment securities held to maturity, amortized cost	148,390	241,676
Investment securities held to maturity, fair value	151,467	248,522
Investment securities available for sale, amortized cost	4,568,346	3,900,751
Investment securities available for sale, fair value	4,638,855	4,063,185
Investment securities held to maturity, amortized cost	306,403	515,598
Investment securities held to maturity, fair value	$ 312,562	$ 529,687