Note 2 - Investments - Security Holdings in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Less than 12 Months, fair value	$ 104,426	$ 58,944
Less than 12 Months, Accumulated Loss	(1,651)	(726)
12 Months or Longer, fair value	374,224	392,045
12 Months or Longer, Accumulated Loss	(34,955)	(33,577)
Fair value	478,650	450,989
Accumulated Loss	(36,606)	(34,303)
US Government Agencies Debt Securities [Member]
Less than 12 Months, fair value	17,385	738
Less than 12 Months, Accumulated Loss	(385)	(1)
12 Months or Longer, fair value	20,957	38,063
12 Months or Longer, Accumulated Loss	(1,002)	(1,415)
Fair value	38,342	38,801
Accumulated Loss	(1,387)	(1,416)
US States and Political Subdivisions Debt Securities [Member]
Less than 12 Months, fair value	512	0
Less than 12 Months, Accumulated Loss	(5)	0
12 Months or Longer, fair value	6,217	6,196
12 Months or Longer, Accumulated Loss	(814)	(839)
Fair value	6,729	6,196
Accumulated Loss	(819)	(839)
Special Revenue Excluding Asset Backed Securities [Member]
Less than 12 Months, fair value	0	0
Less than 12 Months, Accumulated Loss	0	0
12 Months or Longer, fair value	26,380	26,736
12 Months or Longer, Accumulated Loss	(3,415)	(3,149)
Fair value	26,380	26,736
Accumulated Loss	(3,415)	(3,149)
Corporate Debt Securities and Other Debt Obligations [Member]
Less than 12 Months, fair value	41,802	28,872
Less than 12 Months, Accumulated Loss	(402)	(480)
12 Months or Longer, fair value	212,756	204,034
12 Months or Longer, Accumulated Loss	(17,878)	(16,717)
Fair value	254,558	232,906
Accumulated Loss	(18,280)	(17,197)
Asset-Backed Securities [Member]
Less than 12 Months, fair value	44,727	29,334
Less than 12 Months, Accumulated Loss	(859)	(245)
12 Months or Longer, fair value	107,914	117,016
12 Months or Longer, Accumulated Loss	(11,846)	(11,457)
Fair value	152,641	146,350
Accumulated Loss	$ (12,705)	$ (11,702)