Company's Invested Assets and Derivative Instruments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	$ 1,201,866	$ 1,191,186
Equity securities	99,978	98,990
U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	150,118	197,480
Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	5,630
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	54,721	61,243
Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	250,341	358,778
Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	136,893	110,959
Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	172,642	117,593
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	292,383	240,717
Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	139,138	104,416
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,201,866	1,191,186
Equity securities	99,978	98,990
Total invested assets	1,301,844	1,290,176
Total invested liabilities	8,395	16,430
Fair Value, Measurements, Recurring | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	8,395	16,430
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	150,118	197,480
Equity securities	75,750	87,307
Total invested assets	225,868	284,787
Total invested liabilities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,049,216	993,706
Equity securities	23,991	11,683
Total invested assets	1,073,207	1,005,389
Total invested liabilities	8,395	16,430
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	8,395	16,430
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	2,532	0
Equity securities	237	0
Total invested assets	2,769	0
Total invested liabilities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	0	0
Fair Value, Measurements, Recurring | U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	150,118	197,480
Fair Value, Measurements, Recurring | U.S. Treasuries | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	150,118	197,480
Fair Value, Measurements, Recurring | U.S. Treasuries | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0
Fair Value, Measurements, Recurring | U.S. Treasuries | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0
Fair Value, Measurements, Recurring | Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	5,630
Fair Value, Measurements, Recurring | Agency Obligations | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0
Fair Value, Measurements, Recurring | Agency Obligations | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	5,630
Fair Value, Measurements, Recurring | Agency Obligations | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	54,721	61,243
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	54,721	61,243
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	250,341	358,778
Fair Value, Measurements, Recurring | Mortgage Backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Mortgage Backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	250,341	358,778
Fair Value, Measurements, Recurring | Mortgage Backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	136,893	110,959
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	136,893	110,959
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	172,642	117,593
Fair Value, Measurements, Recurring | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	171,686	117,593
Fair Value, Measurements, Recurring | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	956	0
Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	292,383	240,717
Fair Value, Measurements, Recurring | Corporate Bonds | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Corporate Bonds | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	290,807	240,717
Fair Value, Measurements, Recurring | Corporate Bonds | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,576	0
Fair Value, Measurements, Recurring | Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	139,138	104,416
Fair Value, Measurements, Recurring | Foreign Corporate Bonds | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | Foreign Corporate Bonds | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	139,138	104,416
Fair Value, Measurements, Recurring | Foreign Corporate Bonds | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	$ 0	$ 0