Investments - Summary of Securities with Gross Unrealized Loss Positions Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	$ (263)	$ (44)
Estimated Fair Value, Less than Twelve Months	309,813	5,886
Gross Unrealized Loss, Twelve Months or Longer	(4,190)	(4,745)
Estimated Fair Value, Twelve Months or Longer	150,034	200,881
Gross Unrealized Loss, Total	(4,453)	(4,789)
Estimated Fair Value, Total	459,847	206,767
U.S. Treasury and U.S. Government Agencies [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(209)	(22)
Estimated Fair Value, Less than Twelve Months	304,595	3,464
Gross Unrealized Loss, Twelve Months or Longer	(3,289)	(3,778)
Estimated Fair Value, Twelve Months or Longer	131,299	181,463
Gross Unrealized Loss, Total	(3,498)	(3,800)
Estimated Fair Value, Total	435,894	184,927
Corporate Bonds [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(42)	(8)
Estimated Fair Value, Less than Twelve Months	4,736	1,941
Gross Unrealized Loss, Twelve Months or Longer	(901)	(967)
Estimated Fair Value, Twelve Months or Longer	18,735	19,418
Gross Unrealized Loss, Total	(943)	(975)
Estimated Fair Value, Total	23,471	21,359
Exchange-Traded Debt [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(12)	(14)
Estimated Fair Value, Less than Twelve Months	482	481
Gross Unrealized Loss, Twelve Months or Longer	0	0
Estimated Fair Value, Twelve Months or Longer	0	0
Gross Unrealized Loss, Total	(12)	(14)
Estimated Fair Value, Total	$ 482	$ 481