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Fair Values of Financial Instruments - Schedule of Assets Measured at Amortized Cost Net of Allowance For Credit Losses (Detail) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value $ 364,386 $ 399,721
States and Political Subdivisions [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 332,394 356,268
Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 21,151 32,670
U.S. Agency-Based Mortgage-Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 2,422 2,643
U.S. Treasury Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 8,410 8,127
Asset-Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 9 13
Fair Value, Inputs, Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 8,410 8,127
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 8,410 8,127
Fair Value, Inputs, Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 355,976 391,594
Fair Value, Inputs, Level 2 [Member] | States and Political Subdivisions [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 332,394 356,268
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 21,151 32,670
Fair Value, Inputs, Level 2 [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value 2,422 2,643
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Securities held-to-maturity-fixed maturity, Fair Value $ 9 $ 13