Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	$ 3,139,388	$ 2,937,226
Total liabilities measured at fair value	294,080	282,486
Other Invested Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,021
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	348,395	411,594
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,788,972	2,525,632
Total liabilities measured at fair value	294,080	282,486
Fair Value Based on Net Asset Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,021
Fair Value Based on Net Asset Value | Other Invested Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,021
Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,660,417	2,414,210
Fixed maturities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	55,159	67,394
Fixed maturities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,605,258	2,346,816
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	262,030	252,882
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	55,159	67,394
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	206,871	185,488
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	580,324	576,859
Fixed maturities | States, municipalities and political subdivisions | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	580,324	576,859
Fixed maturities | Agency mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	526,069	379,269
Fixed maturities | Agency mortgage-backed securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	526,069	379,269
Fixed maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	27,055	30,465
Fixed maturities | Residential Mortgage Backed Securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	27,055	30,465
Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	288,636	225,012
Fixed maturities | Asset-backed securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	288,636	225,012
Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	165,405	189,713
Fixed maturities | Commercial mortgage-backed securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	165,405	189,713
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	1,007,165	824,459
Fixed maturities | Mortgage-backed and asset-backed securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,007,165	824,459
Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	810,898	760,010
Fixed maturities | Corporate bonds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	810,898	760,010
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	342,677	305,271
Equity Securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	158,963	126,455
Equity Securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	183,714	178,816
Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	134,273	217,745
Short-term investments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	134,273	217,745
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes, at fair value	294,080	282,486
Senior Notes | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes, at fair value	$ 294,080	$ 282,486