Investments - Gross Unrecognized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses, Carrying Amount and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 436,505		$ 483,757
Held-to-Maturity, Allowance for Credit Losses	(136)	$ (149)	(182)
Held-to-Maturity, Carrying Amount	436,369		483,575
Held-to-Maturity, Gross Unrecognized Gains	4,266		4,218
Held-to-Maturity, Gross Unrecognized Losses	(10,282)		(14,701)
Securities held-to-maturity-fixed maturity, Fair Value	$ 430,353		$ 473,092
Investment, Type [Extensible Enumeration]	U.S. Agency-Based Mortgage-Backed Securities [Member]		U.S. Agency-Based Mortgage-Backed Securities [Member]
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 387,702		$ 416,916
Held-to-Maturity, Allowance for Credit Losses	(35)	(36)	(38)
Held-to-Maturity, Carrying Amount	387,667		416,878
Held-to-Maturity, Gross Unrecognized Gains	4,190		4,166
Held-to-Maturity, Gross Unrecognized Losses	(9,006)		(12,074)
Securities held-to-maturity-fixed maturity, Fair Value	382,851		408,970
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	37,453		52,321
Held-to-Maturity, Allowance for Credit Losses	(100)	(111)	(142)
Held-to-Maturity, Carrying Amount	37,353		52,179
Held-to-Maturity, Gross Unrecognized Gains	12
Held-to-Maturity, Gross Unrecognized Losses	(1,094)		(2,231)
Securities held-to-maturity-fixed maturity, Fair Value	36,271		49,948
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	2,877		3,297
Held-to-Maturity, Carrying Amount	2,877		3,297
Held-to-Maturity, Gross Unrecognized Gains	33		25
Held-to-Maturity, Gross Unrecognized Losses	(81)		(123)
Securities held-to-maturity-fixed maturity, Fair Value	2,829		3,199
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	8,456		11,186
Held-to-Maturity, Carrying Amount	8,456		11,186
Held-to-Maturity, Gross Unrecognized Gains	31		26
Held-to-Maturity, Gross Unrecognized Losses	(101)		(273)
Securities held-to-maturity-fixed maturity, Fair Value	8,386		10,939
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	17		37
Held-to-Maturity, Allowance for Credit Losses	(1)	$ (2)	(2)
Held-to-Maturity, Carrying Amount	16		35
Held-to-Maturity, Gross Unrecognized Gains			1
Held-to-Maturity, Gross Unrecognized Losses			0
Securities held-to-maturity-fixed maturity, Fair Value	$ 16		$ 36

Investments - Gross Unrecognized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses, Carrying Amount and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($)
$ in Thousands

Sep. 30, 2024

Jun. 30, 2024

Dec. 31, 2023

Schedule Of Held To Maturity Securities [Line Items]

Held-to-maturity, Amortized Cost, Totals

$ 436,505

$ 483,757

Held-to-Maturity, Allowance for Credit Losses

(136)

$ (149)

(182)

Held-to-Maturity, Carrying Amount