Company's Invested Assets and Derivative Instruments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	$ 1,191,186	$ 1,253,159
Equity securities	98,990	263,104
U.S. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	197,480	156,689
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	61,243	63,838
Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	358,778	328,374
Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	110,959	188,104
Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	117,593	168,537
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	240,717	248,259
Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	104,416	99,358
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,191,186	1,253,159
Equity securities	98,990	263,104
Total invested assets	1,290,176	1,516,263
Total invested liabilities	16,430	10,275
Fair Value, Measurements, Recurring | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	16,430	10,275
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	197,480	156,689
Equity securities	87,307	251,448
Total invested assets	284,787	408,137
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	993,706	1,096,470
Equity securities	11,683	11,656
Total invested assets	1,005,389	1,108,126
Total invested liabilities	16,430	10,275
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	16,430	10,275
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	0	0
Equity securities	0	0
Total invested assets	0	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. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	197,480	156,689
Fair Value, Measurements, Recurring | U.S. Treasury and Agency Obligations | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	197,480	156,689
Fair Value, Measurements, Recurring | U.S. Treasury and Agency Obligations | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	0	0
Fair Value, Measurements, Recurring | U.S. Treasury and Agency Obligations | 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 | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	61,243	63,838
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	61,243	63,838
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	358,778	328,374
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	358,778	328,374
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	110,959	188,104
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	110,959	188,104
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	117,593	168,537
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	117,593	168,537
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	0	0
Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	240,717	248,259
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	240,717	248,259
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	0	0
Fair Value, Measurements, Recurring | Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	104,416	99,358
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	104,416	99,358
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