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, 2024	Dec. 31, 2023	Sep. 30, 2023
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 475,487	$ 483,757
Held-to-Maturity, Allowance for Credit Losses	(165)	(182)	$ (200)
Held-to-Maturity, Carrying Amount	475,322	483,575
Held-to-Maturity, Gross Unrecognized Gains	2,837	4,218
Held-to-Maturity, Gross Unrecognized Losses	(16,478)	(14,701)
Securities held-to-maturity-fixed maturity, Fair Value	$ 461,681	$ 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	$ 412,900	$ 416,916
Held-to-Maturity, Allowance for Credit Losses	(37)	(38)
Held-to-Maturity, Carrying Amount	412,863	416,878
Held-to-Maturity, Gross Unrecognized Gains	2,804	4,166
Held-to-Maturity, Gross Unrecognized Losses	(13,871)	(12,074)
Securities held-to-maturity-fixed maturity, Fair Value	401,796	408,970
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	48,164	52,321
Held-to-Maturity, Allowance for Credit Losses	(126)	(142)
Held-to-Maturity, Carrying Amount	48,038	52,179
Held-to-Maturity, Gross Unrecognized Gains		0
Held-to-Maturity, Gross Unrecognized Losses	(2,078)	(2,231)
Securities held-to-maturity-fixed maturity, Fair Value	45,960	49,948
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,183	3,297
Held-to-Maturity, Carrying Amount	3,183	3,297
Held-to-Maturity, Gross Unrecognized Gains	19	25
Held-to-Maturity, Gross Unrecognized Losses	(150)	(123)
Securities held-to-maturity-fixed maturity, Fair Value	3,052	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	11,209	11,186
Held-to-Maturity, Carrying Amount	11,209	11,186
Held-to-Maturity, Gross Unrecognized Gains	14	26
Held-to-Maturity, Gross Unrecognized Losses	(379)	(273)
Securities held-to-maturity-fixed maturity, Fair Value	10,844	10,939
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	31	37
Held-to-Maturity, Allowance for Credit Losses	(2)	(2)
Held-to-Maturity, Carrying Amount	29	35
Held-to-Maturity, Gross Unrecognized Gains		1
Held-to-Maturity, Gross Unrecognized Losses		0
Securities held-to-maturity-fixed maturity, Fair Value	$ 29	$ 36