Note 4 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Temporarily impaired securities, less than 12 months, fair value	$ 132,580	$ 769,045
Temporarily impaired securities, less than 12 months, unrealized losses	680	39,023
Temporarily impaired securities, 12 months or longer, fair value	964,495	562,156
Temporarily impaired securities, 12 months or longer, unrealized losses	121,345	110,492
Temporarily impaired securities, total fair value	1,097,075	1,331,201
Temporarily impaired securities, total unrealized losses	122,025	149,515
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	49,831	240,500
Temporarily impaired securities, less than 12 months, unrealized losses	20	1,111
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	49,831	240,500
Temporarily impaired securities, total unrealized losses	20	1,111
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	18,301	0
Temporarily impaired securities, less than 12 months, unrealized losses	108	0
Temporarily impaired securities, 12 months or longer, fair value	1,313	1,806
Temporarily impaired securities, 12 months or longer, unrealized losses	122	121
Temporarily impaired securities, total fair value	19,614	1,806
Temporarily impaired securities, total unrealized losses	230	121
Collateralized Mortgage-Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	0	394,123
Temporarily impaired securities, less than 12 months, unrealized losses	0	33,042
Temporarily impaired securities, 12 months or longer, fair value	768,274	452,739
Temporarily impaired securities, 12 months or longer, unrealized losses	106,442	93,941
Temporarily impaired securities, total fair value	768,274	846,862
Temporarily impaired securities, total unrealized losses	106,442	126,983
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value	0	24,427
Temporarily impaired securities, less than 12 months, unrealized losses	0	1,614
Temporarily impaired securities, 12 months or longer, fair value	28,044	6,634
Temporarily impaired securities, 12 months or longer, unrealized losses	3,194	1,877
Temporarily impaired securities, total fair value	28,044	31,061
Temporarily impaired securities, total unrealized losses	3,194	3,491
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	64,448	109,995
Temporarily impaired securities, less than 12 months, unrealized losses	552	3,256
Temporarily impaired securities, 12 months or longer, fair value	166,864	100,977
Temporarily impaired securities, 12 months or longer, unrealized losses	11,587	14,553
Temporarily impaired securities, total fair value	231,312	210,972
Temporarily impaired securities, total unrealized losses	$ 12,139	$ 17,809