Note 3 - Investments - Security Holdings in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Less than 12 Months, fair value	$ 58,944	$ 384,693
Less than 12 Months, Accumulated Loss	(726)	(31,558)
12 Months or Longer, fair value	392,045	99,986
12 Months or Longer, Accumulated Loss	(33,577)	(15,365)
Fair value	450,989	484,679
Accumulated Loss	(34,303)	(46,923)
US Government Agencies Debt Securities [Member]
Less than 12 Months, fair value	738	41,077
Less than 12 Months, Accumulated Loss	(1)	(1,523)
12 Months or Longer, fair value	38,063	6,853
12 Months or Longer, Accumulated Loss	(1,415)	(730)
Fair value	38,801	47,930
Accumulated Loss	(1,416)	(2,253)
US States and Political Subdivisions Debt Securities [Member]
Less than 12 Months, fair value	0	7,525
Less than 12 Months, Accumulated Loss	0	(1,173)
12 Months or Longer, fair value	6,196	0
12 Months or Longer, Accumulated Loss	(839)	0
Fair value	6,196	7,525
Accumulated Loss	(839)	(1,173)
Special Revenue Excluding Asset Backed Securities [Member]
Less than 12 Months, fair value	0	25,091
Less than 12 Months, Accumulated Loss	0	(3,287)
12 Months or Longer, fair value	26,736	5,080
12 Months or Longer, Accumulated Loss	(3,149)	(1,200)
Fair value	26,736	30,171
Accumulated Loss	(3,149)	(4,487)
Corporate Debt Securities and Other Debt Obligations [Member]
Less than 12 Months, fair value	28,872	192,185
Less than 12 Months, Accumulated Loss	(480)	(15,667)
12 Months or Longer, fair value	204,034	55,605
12 Months or Longer, Accumulated Loss	(16,717)	(8,481)
Fair value	232,906	247,790
Accumulated Loss	(17,197)	(24,148)
Asset-Backed Securities [Member]
Less than 12 Months, fair value	29,334	118,815
Less than 12 Months, Accumulated Loss	(245)	(9,908)
12 Months or Longer, fair value	117,016	32,448
12 Months or Longer, Accumulated Loss	(11,457)	(4,954)
Fair value	146,350	151,263
Accumulated Loss	$ (11,702)	$ (14,862)