Investments - Summary of Securities with Gross Unrealized Loss Positions Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	$ (405)	$ (44)
Estimated Fair Value, Less than Twelve Months	285,876	5,886
Gross Unrealized Loss, Twelve Months or Longer	(3,564)	(4,745)
Estimated Fair Value, Twelve Months or Longer	151,899	200,881
Gross Unrealized Loss, Total	(3,969)	(4,789)
Estimated Fair Value, Total	437,775	206,767
Fixed-Maturity Securities [Member] | U.S. Treasury and U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(351)	(22)
Estimated Fair Value, Less than Twelve Months	279,112	3,464
Gross Unrealized Loss, Twelve Months or Longer	(2,648)	(3,778)
Estimated Fair Value, Twelve Months or Longer	131,989	181,463
Gross Unrealized Loss, Total	(2,999)	(3,800)
Estimated Fair Value, Total	411,101	184,927
Fixed-Maturity Securities [Member] | Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(32)	(8)
Estimated Fair Value, Less than Twelve Months	6,291	1,941
Gross Unrealized Loss, Twelve Months or Longer	(916)	(967)
Estimated Fair Value, Twelve Months or Longer	19,910	19,418
Gross Unrealized Loss, Total	(948)	(975)
Estimated Fair Value, Total	26,201	21,359
Fixed-Maturity Securities [Member] | Exchange-Traded Debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(22)	(14)
Estimated Fair Value, Less than Twelve Months	473	481
Gross Unrealized Loss, Twelve Months or Longer	0	0
Estimated Fair Value, Twelve Months or Longer	0	0
Gross Unrealized Loss, Total	(22)	(14)
Estimated Fair Value, Total	$ 473	$ 481