Summary Of Investments Other Than Investments In Related Parties (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 1,426,975	[1]
Value	1,426,709
Amount Included in the Balance Sheet	1,426,709
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,241,339	[1]
Value	1,240,031
Amount Included in the Balance Sheet	1,240,031
Fixed Maturities | U.S. Treasury and Agency Obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	71,517	[1]
Value	72,047
Amount Included in the Balance Sheet	72,047
Fixed Maturities | Obligations of States and Political Subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	155,402	[1]
Value	156,446
Amount Included in the Balance Sheet	156,446
Fixed Maturities | Mortgage Backed And Asset Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	506,842	[1]
Value	505,651
Amount Included in the Balance Sheet	505,651
Fixed Maturities | Public Utility, Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	24,949	[1]
Value	24,847
Amount Included in the Balance Sheet	24,847
Fixed Maturities | All Other Corporate Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	482,629	[1]
Value	481,040
Amount Included in the Balance Sheet	481,040
Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	119,515	[1]
Value	120,557
Amount Included in the Balance Sheet	120,557
Equity securities | Public Utility, Equities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	8,881	[1]
Value	9,146
Amount Included in the Balance Sheet	9,146
Equity securities | Industrial, Miscellaneous, and All Others
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	110,634	[1]
Value	111,411
Amount Included in the Balance Sheet	111,411
Other long-term investments | Other Invested Assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	66,121	[1]
Value	66,121
Amount Included in the Balance Sheet	$ 66,121

[1] Original cost of equity securities; original cost of fixed maturities adjusted for amortization of premiums and accretion of discounts; original cost for other long-term investments adjusted for income or loss earned on investments in accordance with equity method of accounting. All amounts are shown net of impairment losses.