Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	$ 1,983,918	$ 1,969,503
Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,888,069	1,882,245
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	336,070	324,145
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	410,836	392,250
Fixed maturities | Agency mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	395,860	382,628
Fixed maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	23,148	20,463
Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	48,934	46,093
Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	216,034	190,015
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	683,976	639,199
Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	457,187	526,651
Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	95,849	87,258
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	232,474	300,191
Level 1 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	136,625	212,933
Level 1 | Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	136,625	212,933
Level 1 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	95,849	87,258
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,751,444	1,667,475
Level 2 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,751,444	1,667,475
Level 2 | Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	199,445	111,212
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	410,836	392,250
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	395,860	382,628
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	23,148	20,463
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	48,934	46,093
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	216,034	188,178
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	683,976	637,362
Level 2 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	457,187	526,651
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value		1,837
Level 3 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value		1,837
Level 3 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value		1,837
Level 3 | 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		$ 1,837