Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	$ 2,710,763	$ 2,729,735
Liabilities, at fair Value	282,502	285,710
Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,313,937	2,365,934
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	295,687	397,923
Fixed maturities | States, municipalities and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	565,188	541,007
Fixed maturities | Agency mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	343,916	364,622
Fixed maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	32,396	34,087
Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	195,512	206,413
Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	216,185	206,318
Fixed maturities | Mortgage-backed and asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	788,009	811,440
Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	665,053	615,564
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	195,078	184,295
Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	201,748	179,506
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, at fair Value	282,502	285,710
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	404,823	453,593
Level 1 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	80,425	146,904
Level 1 | Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	80,425	146,904
Level 1 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	122,650	127,183
Level 1 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	201,748	179,506
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,305,940	2,276,142
Liabilities, at fair Value	282,502	285,710
Level 2 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,233,512	2,219,030
Level 2 | Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	215,262	251,019
Level 2 | Fixed maturities | States, municipalities and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	565,188	541,007
Level 2 | Fixed maturities | Agency mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	343,916	364,622
Level 2 | Fixed maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	32,396	34,087
Level 2 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	195,512	206,413
Level 2 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	216,185	206,318
Level 2 | Fixed maturities | Mortgage-backed and asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	788,009	811,440
Level 2 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	665,053	615,564
Level 2 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	72,428	57,112
Level 2 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, at fair Value	$ 282,502	$ 285,710