Investments - Gross Unrealized 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, 2026	Jun. 30, 2026	Dec. 31, 2025
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 323,716		$ 350,160
Held-to-Maturity, Allowance for Credit Losses	(58)	$ (60)	(73)
Held-to-maturity, Carrying Amount, Totals	323,658		350,087
Held-to-Maturity, Gross Unrealized Gains	61		2,062
Held-to-Maturity, Gross Unrealized Losses	(23,870)		(7,573)
Securities held-to-maturity-fixed maturity, Fair Value	$ 299,849		$ 344,576
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	$ 297,291		$ 322,430
Held-to-Maturity, Allowance for Credit Losses	(20)	(21)	(23)
Held-to-maturity, Carrying Amount, Totals	297,271		322,407
Held-to-Maturity, Gross Unrealized Gains	59		2,030
Held-to-Maturity, Gross Unrealized Losses	(22,705)		(6,908)
Securities held-to-maturity-fixed maturity, Fair Value	274,625		317,529
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	15,914		16,751
Held-to-Maturity, Allowance for Credit Losses	(38)	(39)	(50)
Held-to-maturity, Carrying Amount, Totals	15,876		16,701
Held-to-Maturity, Gross Unrealized Gains	2		0
Held-to-Maturity, Gross Unrealized Losses	(595)		(456)
Securities held-to-maturity-fixed maturity, Fair Value	15,283		16,245
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	2,115		2,403
Held-to-maturity, Carrying Amount, Totals	2,115		2,403
Held-to-Maturity, Gross Unrealized Gains	0		26
Held-to-Maturity, Gross Unrealized Losses	(181)		(81)
Securities held-to-maturity-fixed maturity, Fair Value	1,934		2,348
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	8,387		8,567
Held-to-maturity, Carrying Amount, Totals	8,387		8,567
Held-to-Maturity, Gross Unrealized Gains	0		6
Held-to-Maturity, Gross Unrealized Losses	(389)		(128)
Securities held-to-maturity-fixed maturity, Fair Value	7,998		8,445
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	9		9
Held-to-Maturity, Allowance for Credit Losses	0	$ 0	0
Held-to-maturity, Carrying Amount, Totals	9		9
Securities held-to-maturity-fixed maturity, Fair Value	$ 9		$ 9

Investments - Gross Unrealized 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, 2026

Jun. 30, 2026

Dec. 31, 2025

Schedule Of Held To Maturity Securities [Line Items]

Held-to-maturity, Amortized Cost, Totals

$ 323,716

$ 350,160

Held-to-Maturity, Allowance for Credit Losses

(58)

$ (60)

(73)

Held-to-maturity, Carrying Amount, Totals