Investments - Summary of Securities with Gross Unrealized Loss Positions Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	$ (5,226)	$ (146)
Estimated Fair Value, Less than Twelve Months	372,801	14,703
Gross Unrealized Loss, Twelve Months or Longer	(98)	(13)
Estimated Fair Value, Twelve Months or Longer	2,048	616
Gross Unrealized Loss, Total	(5,324)	(159)
Estimated Fair Value, Total	374,849	15,319
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	(4,664)	(73)
Estimated Fair Value, Less than Twelve Months	348,459	9,809
Gross Unrealized Loss, Twelve Months or Longer	(98)	(13)
Estimated Fair Value, Twelve Months or Longer	2,048	616
Gross Unrealized Loss, Total	(4,762)	(86)
Estimated Fair Value, Total	350,507	10,425
Corporate Bonds [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(556)	(53)
Estimated Fair Value, Less than Twelve Months	23,828	4,452
Gross Unrealized Loss, Twelve Months or Longer	0	0
Estimated Fair Value, Twelve Months or Longer	0	0
Gross Unrealized Loss, Total	(556)	(53)
Estimated Fair Value, Total	23,828	4,452
Exchange-Traded Debt [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(1)
Estimated Fair Value, Less than Twelve Months	23
Gross Unrealized Loss, Twelve Months or Longer	0
Estimated Fair Value, Twelve Months or Longer	0
Gross Unrealized Loss, Total	(1)
Estimated Fair Value, Total	23
Redeemable Preferred Stock [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(3)	(20)
Estimated Fair Value, Less than Twelve Months	104	442
Gross Unrealized Loss, Twelve Months or Longer	0	0
Estimated Fair Value, Twelve Months or Longer	0	0
Gross Unrealized Loss, Total	(3)	(20)
Estimated Fair Value, Total	104	$ 442
State, Municipalities, and Political Subdivisions [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Loss, Less than Twelve Months	(2)
Estimated Fair Value, Less than Twelve Months	387
Gross Unrealized Loss, Twelve Months or Longer	0
Estimated Fair Value, Twelve Months or Longer	0
Gross Unrealized Loss, Total	(2)
Estimated Fair Value, Total	$ 387