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	Mar. 31, 2025	Dec. 31, 2024
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 399,341	$ 413,177
Held-to-Maturity, Allowance for Credit Losses	(100)	(116)
Held-to-maturity, Carrying Amount, Totals	399,241	413,061
Held-to-Maturity, Gross Unrecognized Gains	1,289	1,838
Held-to-Maturity, Gross Unrecognized Losses	(14,549)	(15,178)
Securities held-to-maturity-fixed maturity, Fair Value	$ 385,981	$ 399,721
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	$ 360,026	$ 368,056
Held-to-Maturity, Allowance for Credit Losses	(29)	(30)
Held-to-maturity, Carrying Amount, Totals	359,997	368,026
Held-to-Maturity, Gross Unrecognized Gains	1,250	1,810
Held-to-Maturity, Gross Unrecognized Losses	(13,333)	(13,568)
Securities held-to-maturity-fixed maturity, Fair Value	347,914	356,268
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	28,110	33,849
Held-to-Maturity, Allowance for Credit Losses	(71)	(86)
Held-to-maturity, Carrying Amount, Totals	28,039	33,763
Held-to-Maturity, Gross Unrecognized Gains	10	6
Held-to-Maturity, Gross Unrecognized Losses	(888)	(1,099)
Securities held-to-maturity-fixed maturity, Fair Value	27,161	32,670
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	2,696	2,781
Held-to-maturity, Carrying Amount, Totals	2,696	2,781
Held-to-Maturity, Gross Unrecognized Gains	19	11
Held-to-Maturity, Gross Unrecognized Losses	(120)	(149)
Securities held-to-maturity-fixed maturity, Fair Value	2,595	2,643
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	8,500	8,478
Held-to-maturity, Carrying Amount, Totals	8,500	8,478
Held-to-Maturity, Gross Unrecognized Gains	10	11
Held-to-Maturity, Gross Unrecognized Losses	(208)	(362)
Securities held-to-maturity-fixed maturity, Fair Value	8,302	8,127
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	9	13
Held-to-maturity, Carrying Amount, Totals	9	13
Securities held-to-maturity-fixed maturity, Fair Value	$ 9	$ 13