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	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 464,983		$ 483,757
Held-to-Maturity, Allowance for Credit Losses	(149)	$ (165)	(182)
Held-to-Maturity, Carrying Amount	464,834		483,575
Held-to-Maturity, Gross Unrecognized Gains	2,275		4,218
Held-to-Maturity, Gross Unrecognized Losses	(18,501)		(14,701)
Securities held-to-maturity-fixed maturity, Fair Value	$ 448,608		$ 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	$ 402,886		$ 416,916
Held-to-Maturity, Allowance for Credit Losses	(36)	(37)	(38)
Held-to-Maturity, Carrying Amount	402,850		416,878
Held-to-Maturity, Gross Unrecognized Gains	2,253		4,166
Held-to-Maturity, Gross Unrecognized Losses	(15,940)		(12,074)
Securities held-to-maturity-fixed maturity, Fair Value	389,163		408,970
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	47,792		52,321
Held-to-Maturity, Allowance for Credit Losses	(111)	(126)	(142)
Held-to-Maturity, Carrying Amount	47,681		52,179
Held-to-Maturity, Gross Unrecognized Losses	(1,997)		(2,231)
Securities held-to-maturity-fixed maturity, Fair Value	45,684		49,948
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,047		3,297
Held-to-Maturity, Carrying Amount	3,047		3,297
Held-to-Maturity, Gross Unrecognized Gains	9		25
Held-to-Maturity, Gross Unrecognized Losses	(164)		(123)
Securities held-to-maturity-fixed maturity, Fair Value	2,892		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	11,233		11,186
Held-to-Maturity, Carrying Amount	11,233		11,186
Held-to-Maturity, Gross Unrecognized Gains	13		26
Held-to-Maturity, Gross Unrecognized Losses	(400)		(273)
Securities held-to-maturity-fixed maturity, Fair Value	10,846		10,939
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	25		37
Held-to-Maturity, Allowance for Credit Losses	(2)	$ (2)	(2)
Held-to-Maturity, Carrying Amount	23		35
Held-to-Maturity, Gross Unrecognized Gains			1
Held-to-Maturity, Gross Unrecognized Losses			0
Securities held-to-maturity-fixed maturity, Fair Value	$ 23		$ 36