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) (Detail) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 483,757,000	$ 491,927,000
Held-to-Maturity, Allowance for Credit Losses	(182,000)	(239,000)
Held-to-Maturity, Carrying Amount	483,575,000	491,688,000
Held-to-Maturity, Gross Unrecognized Gains	4,218,000	983,000
Held-to-Maturity, Gross Unrecognized Losses	(14,701,000)	(24,527,000)
Securities held-to-maturity-fixed maturity, Fair Value	$ 473,092,000	$ 468,144,000
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	$ 416,916,000	$ 415,136,000
Held-to-Maturity, Allowance for Credit Losses	(38,000)	(40,000)
Held-to-Maturity, Carrying Amount	416,878,000	415,096,000
Held-to-Maturity, Gross Unrecognized Gains	4,166,000	922,000
Held-to-Maturity, Gross Unrecognized Losses	(12,074,000)	(20,074,000)
Securities held-to-maturity-fixed maturity, Fair Value	408,970,000	395,944,000
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	52,321,000	59,903,000
Held-to-Maturity, Allowance for Credit Losses	(142,000)	(196,000)
Held-to-Maturity, Carrying Amount	52,179,000	59,707,000
Held-to-Maturity, Gross Unrecognized Gains		1,000
Held-to-Maturity, Gross Unrecognized Losses	(2,231,000)	(3,857,000)
Securities held-to-maturity-fixed maturity, Fair Value	49,948,000	55,851,000
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,297,000	3,696,000
Held-to-Maturity, Carrying Amount	3,297,000	3,696,000
Held-to-Maturity, Gross Unrecognized Gains	25,000	33,000
Held-to-Maturity, Gross Unrecognized Losses	(123,000)	(153,000)
Securities held-to-maturity-fixed maturity, Fair Value	3,199,000	3,576,000
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	11,186,000	13,123,000
Held-to-Maturity, Carrying Amount	11,186,000	13,123,000
Held-to-Maturity, Gross Unrecognized Gains	26,000	25,000
Held-to-Maturity, Gross Unrecognized Losses	(273,000)	(442)
Securities held-to-maturity-fixed maturity, Fair Value	10,939,000	12,706,000
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	37,000	69,000
Held-to-Maturity, Allowance for Credit Losses	(2,000)	(3,000)
Held-to-Maturity, Carrying Amount	35,000	66,000
Held-to-Maturity, Gross Unrecognized Gains	1,000	2,000
Held-to-Maturity, Gross Unrecognized Losses		(1,000)
Securities held-to-maturity-fixed maturity, Fair Value	$ 36,000	$ 67,000