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, 2025	Jun. 30, 2025	Dec. 31, 2024
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 372,098		$ 413,177
Held-to-Maturity, Allowance for Credit Losses	(80)	$ (88)	(116)
Held-to-maturity, Carrying Amount, Totals	372,018		413,061
Held-to-Maturity, Gross Unrecognized Gains	1,436		1,838
Held-to-Maturity, Gross Unrecognized Losses	(9,068)		(15,178)
Securities held-to-maturity-fixed maturity, Fair Value	$ 364,386		$ 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	$ 339,321		$ 368,056
Held-to-Maturity, Allowance for Credit Losses	(24)	(27)	(30)
Held-to-maturity, Carrying Amount, Totals	339,297		368,026
Held-to-Maturity, Gross Unrecognized Gains	1,399		1,810
Held-to-Maturity, Gross Unrecognized Losses	(8,302)		(13,568)
Securities held-to-maturity-fixed maturity, Fair Value	332,394		356,268
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	21,733		33,849
Held-to-Maturity, Allowance for Credit Losses	(56)	(61)	(86)
Held-to-maturity, Carrying Amount, Totals	21,677		33,763
Held-to-Maturity, Gross Unrecognized Gains	4		6
Held-to-Maturity, Gross Unrecognized Losses	(530)		(1,099)
Securities held-to-maturity-fixed maturity, Fair Value	21,151		32,670
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	2,490		2,781
Held-to-maturity, Carrying Amount, Totals	2,490		2,781
Held-to-Maturity, Gross Unrecognized Gains	25		11
Held-to-Maturity, Gross Unrecognized Losses	(93)		(149)
Securities held-to-maturity-fixed maturity, Fair Value	2,422		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	8,545		8,478
Held-to-maturity, Carrying Amount, Totals	8,545		8,478
Held-to-Maturity, Gross Unrecognized Gains	8		11
Held-to-Maturity, Gross Unrecognized Losses	(143)		(362)
Securities held-to-maturity-fixed maturity, Fair Value	8,410		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, Allowance for Credit Losses	0	$ 0	0
Held-to-maturity, Carrying Amount, Totals	9		13
Securities held-to-maturity-fixed maturity, Fair Value	$ 9		$ 13

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, 2025

Jun. 30, 2025

Dec. 31, 2024

Schedule Of Held To Maturity Securities [Line Items]

Held-to-maturity, Amortized Cost, Totals

$ 372,098

$ 413,177

Held-to-Maturity, Allowance for Credit Losses

(80)

$ (88)

(116)

Held-to-maturity, Carrying Amount, Totals