Note 4 - Investment Securities - Continuous Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Unrealized loss position, fair value, less than 12 months	$ 26,251	$ 241,572
Unrealized loss position, unrealized losses, less than 12 months	(1,697)	(33,323)
Unrealized loss position, fair value, more than 12 months	557,761	361,297
Unrealized loss position, unrealized losses, more than 12 months	(85,223)	(70,521)
Unrealized loss position, fair value	584,012	602,869
Unrealized loss position, unrealized losses	$ (86,920)	$ (103,844)
Number of securities, less than 12 months	45	272
Number of securities, more than 12 months	390	213
Number of securities	435	485
US Government Agencies Debt Securities [Member]
Unrealized loss position, fair value, less than 12 months	$ 0	$ 9,285
Unrealized loss position, unrealized losses, less than 12 months	0	(377)
Unrealized loss position, fair value, more than 12 months	99,996	93,312
Unrealized loss position, unrealized losses, more than 12 months	(11,684)	(13,409)
Unrealized loss position, fair value	99,996	102,597
Unrealized loss position, unrealized losses	$ (11,684)	(13,786)
Number of securities	46
US States and Political Subdivisions Debt Securities [Member]
Unrealized loss position, fair value, less than 12 months	$ 19,043	170,484
Unrealized loss position, unrealized losses, less than 12 months	(1,317)	(26,928)
Unrealized loss position, fair value, more than 12 months	254,498	112,353
Unrealized loss position, unrealized losses, more than 12 months	(40,452)	(25,835)
Unrealized loss position, fair value	273,541	282,837
Unrealized loss position, unrealized losses	$ (41,769)	(52,763)
Number of securities	253
Mortgage-Backed Securities, Issued by US Government Sponsored Enterprises [Member]
Unrealized loss position, fair value, less than 12 months	$ 7,208	61,803
Unrealized loss position, unrealized losses, less than 12 months	(380)	(6,018)
Unrealized loss position, fair value, more than 12 months	203,267	155,632
Unrealized loss position, unrealized losses, more than 12 months	(33,087)	(31,277)
Unrealized loss position, fair value	210,475	217,435
Unrealized loss position, unrealized losses	$ (33,467)	$ (37,295)
Number of securities	136