Note 7 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Temporarily impaired securities, less than 12 months, fair value	$ 525,046	$ 132,580
Temporarily impaired securities, less than 12 months, unrealized losses	718	680
Temporarily impaired securities, 12 months or longer, fair value	934,725	964,495
Temporarily impaired securities, 12 months or longer, unrealized losses	130,192	121,345
Temporarily impaired securities, total fair value	1,459,771	1,097,075
Temporarily impaired securities, total unrealized losses	130,910	122,025
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	392,376	49,831
Temporarily impaired securities, less than 12 months, unrealized losses	120	20
Temporarily impaired securities, 12 months or longer, fair value	0	0
Temporarily impaired securities, 12 months or longer, unrealized losses	0	0
Temporarily impaired securities, total fair value	392,376	49,831
Temporarily impaired securities, total unrealized losses	120	20
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	1,471	18,301
Temporarily impaired securities, less than 12 months, unrealized losses	2	108
Temporarily impaired securities, 12 months or longer, fair value	2,524	1,313
Temporarily impaired securities, 12 months or longer, unrealized losses	124	122
Temporarily impaired securities, total fair value	3,995	19,614
Temporarily impaired securities, total unrealized losses	126	230
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	59,745
Temporarily impaired securities, less than 12 months, unrealized losses	40
Temporarily impaired securities, 12 months or longer, fair value	0
Temporarily impaired securities, 12 months or longer, unrealized losses	0
Temporarily impaired securities, total fair value	59,745
Temporarily impaired securities, total unrealized losses	40
Collateralized Mortgage-Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	16,923	0
Temporarily impaired securities, less than 12 months, unrealized losses	87	0
Temporarily impaired securities, 12 months or longer, fair value	736,362	768,274
Temporarily impaired securities, 12 months or longer, unrealized losses	117,045	106,442
Temporarily impaired securities, total fair value	753,285	768,274
Temporarily impaired securities, total unrealized losses	117,132	106,442
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value	0	0
Temporarily impaired securities, less than 12 months, unrealized losses	0	0
Temporarily impaired securities, 12 months or longer, fair value	27,198	28,044
Temporarily impaired securities, 12 months or longer, unrealized losses	3,296	3,194
Temporarily impaired securities, total fair value	27,198	28,044
Temporarily impaired securities, total unrealized losses	3,296	3,194
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	54,531	64,448
Temporarily impaired securities, less than 12 months, unrealized losses	469	552
Temporarily impaired securities, 12 months or longer, fair value	168,641	166,864
Temporarily impaired securities, 12 months or longer, unrealized losses	9,727	11,587
Temporarily impaired securities, total fair value	223,172	231,312
Temporarily impaired securities, total unrealized losses	$ 10,196	$ 12,139