Company's Invested Assets and Derivative Instruments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands		Sep. 30, 2019		Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		$ 1,234,435		$ 1,235,155
Equity securities		262,367		124,747
U.S. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		165,482		78,855
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		56,032		95,613
Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		206,387		117,854
Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		244,853		202,722
Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		183,362		183,754
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		274,738		440,855
Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		103,581		115,502
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		1,234,435		1,235,155
Equity securities		262,367		124,747
Total invested assets		1,496,802	[1]	1,359,902	[2]
Total invested liabilities		12,422		4,062
Fair Value, Measurements, Recurring | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities		12,422		4,062
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		165,482		78,855
Equity securities		262,367		124,747
Total invested assets		427,849	[1]	203,602	[2]
Total invested liabilities		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
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		1,068,953		1,156,300
Equity securities		0
Total invested assets		1,068,953	[1]	1,156,300	[2]
Total invested liabilities		12,422		4,062
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		12,422		4,062
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
Equity securities		0
Total invested assets	[1]	0
Total invested liabilities		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
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		165,482		78,855
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		165,482		78,855
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
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
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		56,032		95,613
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
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		56,032		95,613
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
Fair Value, Measurements, Recurring | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		206,387		117,854
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
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		206,387		117,854
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
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		244,853		202,722
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
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		244,853		202,722
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
Fair Value, Measurements, Recurring | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		183,362		183,754
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
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		183,362		183,754
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
Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		274,738		440,855
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
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		274,738		440,855
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
Fair Value, Measurements, Recurring | Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets		103,581		115,502
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
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		103,581		$ 115,502
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

[1] Excluded from the table above are limited partnerships of $40.1 million at September 30, 2019 whose fair value is based on net asset value as a practical expedient.

[2] Excluded from the table above are limited partnerships of $50.8 million at December 31, 2018 whose fair value is based on net asset value as a practical expedient.