Note 8 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Temporarily impaired securities, less than 12 months, fair value	$ 196,414	$ 769,045
Temporarily impaired securities, less than 12 months, unrealized losses	2,649	39,023
Temporarily impaired securities, 12 months or longer, fair value	986,143	562,156
Temporarily impaired securities, 12 months or longer, unrealized losses	173,731	110,492
Temporarily impaired securities, total fair value	1,182,557	1,331,201
Temporarily impaired securities, total unrealized losses	176,380	149,515
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	49,153	240,500
Temporarily impaired securities, less than 12 months, unrealized losses	126	1,111
Temporarily impaired securities, 12 months or longer, fair value	49,804	0
Temporarily impaired securities, 12 months or longer, unrealized losses	29	0
Temporarily impaired securities, total fair value	98,957	240,500
Temporarily impaired securities, total unrealized losses	155	1,111
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	17,698	0
Temporarily impaired securities, less than 12 months, unrealized losses	39	0
Temporarily impaired securities, 12 months or longer, fair value	1,487	1,806
Temporarily impaired securities, 12 months or longer, unrealized losses	122	121
Temporarily impaired securities, total fair value	19,185	1,806
Temporarily impaired securities, total unrealized losses	161	121
Collateralized Mortgage-Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	17,695	394,123
Temporarily impaired securities, less than 12 months, unrealized losses	642	33,042
Temporarily impaired securities, 12 months or longer, fair value	743,568	452,739
Temporarily impaired securities, 12 months or longer, unrealized losses	154,316	93,941
Temporarily impaired securities, total fair value	761,263	846,862
Temporarily impaired securities, total unrealized losses	154,958	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	27,698	6,634
Temporarily impaired securities, 12 months or longer, unrealized losses	4,317	1,877
Temporarily impaired securities, total fair value	27,698	31,061
Temporarily impaired securities, total unrealized losses	4,317	3,491
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	68,285	109,995
Temporarily impaired securities, less than 12 months, unrealized losses	1,715	3,256
Temporarily impaired securities, 12 months or longer, fair value	163,586	100,977
Temporarily impaired securities, 12 months or longer, unrealized losses	14,947	14,553
Temporarily impaired securities, total fair value	231,871	210,972
Temporarily impaired securities, total unrealized losses	16,662	$ 17,809
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	33,621
Temporarily impaired securities, less than 12 months, unrealized losses	89
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	33,621
Temporarily impaired securities, total unrealized losses	89
Debt Security, Corporate, Non-US [Member]
Temporarily impaired securities, less than 12 months, fair value	9,962
Temporarily impaired securities, less than 12 months, unrealized losses	38
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	9,962
Temporarily impaired securities, total unrealized losses	$ 38