Note 8 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Temporarily impaired securities, less than 12 months, fair value	$ 599,405	$ 769,045
Temporarily impaired securities, less than 12 months, unrealized losses	10,982	39,023
Temporarily impaired securities, 12 months or longer, fair value	816,890	562,156
Temporarily impaired securities, 12 months or longer, unrealized losses	131,299	110,492
Temporarily impaired securities, total fair value	1,416,295	1,331,201
Temporarily impaired securities, total unrealized losses	142,281	149,515
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	272,038	240,500
Temporarily impaired securities, less than 12 months, unrealized losses	447	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	272,038	240,500
Temporarily impaired securities, total unrealized losses	447	1,111
US Government Corporations and Agencies Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	15,885	0
Temporarily impaired securities, less than 12 months, unrealized losses	8	0
Temporarily impaired securities, 12 months or longer, fair value	1,542	1,806
Temporarily impaired securities, 12 months or longer, unrealized losses	124	121
Temporarily impaired securities, total fair value	17,427	1,806
Temporarily impaired securities, total unrealized losses	132	121
US Government-sponsored Enterprises Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	33,614
Temporarily impaired securities, less than 12 months, unrealized losses	95
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	33,614
Temporarily impaired securities, total unrealized losses	95
Collateralized Mortgage-Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	122,348	394,123
Temporarily impaired securities, less than 12 months, unrealized losses	6,039	33,042
Temporarily impaired securities, 12 months or longer, fair value	689,736	452,739
Temporarily impaired securities, 12 months or longer, unrealized losses	115,033	93,941
Temporarily impaired securities, total fair value	812,084	846,862
Temporarily impaired securities, total unrealized losses	121,072	126,983
Collateralized Mortgage Obligations [Member]
Temporarily impaired securities, less than 12 months, fair value	23,251	24,427
Temporarily impaired securities, less than 12 months, unrealized losses	1,662	1,614
Temporarily impaired securities, 12 months or longer, fair value	6,252	6,634
Temporarily impaired securities, 12 months or longer, unrealized losses	1,886	1,877
Temporarily impaired securities, total fair value	29,503	31,061
Temporarily impaired securities, total unrealized losses	3,548	3,491
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	122,562	109,995
Temporarily impaired securities, less than 12 months, unrealized losses	2,438	3,256
Temporarily impaired securities, 12 months or longer, fair value	119,360	100,977
Temporarily impaired securities, 12 months or longer, unrealized losses	14,256	14,553
Temporarily impaired securities, total fair value	241,922	210,972
Temporarily impaired securities, total unrealized losses	16,694	$ 17,809
Debt Security, Corporate, Non-US [Member]
Temporarily impaired securities, less than 12 months, fair value	9,707
Temporarily impaired securities, less than 12 months, unrealized losses	293
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	9,707
Temporarily impaired securities, total unrealized losses	$ 293