Fair Value Measurements - Schedule of Assets Measured at Amortized Cost Net of Allowance For Credit Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 631,688	$ 621,343
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	505,020	485,647
Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	101,164	110,925
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	11,072	11,511
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	9,061	7,873
Obligations of U.S. Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	5,173	5,168
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	198	219
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	9,061	7,873
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	9,061	7,873
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	622,627	613,470
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	505,020	485,647
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	101,164	110,925
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	11,072	11,511
Fair Value, Inputs, Level 2 [Member] | Obligations of U.S. Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	5,173	5,168
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	$ 198	$ 219