Fixed Maturities And Equity Securities By Asset Class That Are Measured At Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	$ 2,128,078	$ 1,983,918
Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	2,024,429	1,888,069
Fixed maturities \| U.S. 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	454,523	336,070
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	426,218	410,836
Fixed maturities \| Agency mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	398,310	395,860
Fixed maturities \| Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	37,805	23,148
Fixed maturities \| Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	54,416	48,934
Fixed maturities \| Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	209,273	216,034
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	699,804	683,976
Fixed maturities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	443,884	457,187
Equity Securities \| Common Stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	103,649	95,849
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	277,886	232,474
Level 1 \| Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	174,237	136,625
Level 1 \| Fixed maturities \| U.S. 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	174,237	136,625
Level 1 \| Equity Securities \| Common Stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	103,649	95,849
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,850,192	1,751,444
Level 2 \| Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,850,192	1,751,444
Level 2 \| Fixed maturities \| U.S. 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	280,286	199,445
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	426,218	410,836
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	398,310	395,860
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	37,805	23,148
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	54,416	48,934
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	209,273	216,034
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	699,804	683,976
Level 2 \| Fixed maturities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	$ 443,884	$ 457,187

Fixed Maturities And Equity Securities By Asset Class That Are Measured At Fair Value (Detail) (USD $)
In Thousands, unless otherwise specified

Jun. 30, 2012

Dec. 31, 2011

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

$ 2,128,078

$ 1,983,918

Fixed maturities

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

2,024,429

1,888,069

Fixed maturities | U.S. 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

454,523

336,070

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

426,218

410,836

Fixed maturities | Agency mortgage-backed securities

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

398,310

395,860

Fixed maturities | Residential Mortgage Backed Securities

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

37,805

23,148

Fixed maturities | Asset-backed securities

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

54,416

48,934

Fixed maturities | Commercial mortgage-backed securities

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

209,273

216,034

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

699,804

683,976

Fixed maturities | Corporate bonds

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

443,884

457,187

Equity Securities | Common Stock

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

103,649

95,849

Level 1

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

277,886

232,474

Level 1 | Fixed maturities

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

174,237

136,625

Level 1 | Fixed maturities | U.S. 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

174,237

136,625

Level 1 | Equity Securities | Common Stock

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

103,649

95,849

Level 2

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

1,850,192

1,751,444

Level 2 | Fixed maturities

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

1,850,192

1,751,444

Level 2 | Fixed maturities | U.S. 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

280,286

199,445

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

426,218

410,836

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

398,310

395,860

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

37,805

23,148

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

54,416

48,934

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

209,273

216,034

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

699,804

683,976

Level 2 | Fixed maturities | Corporate bonds

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fixed maturities and equity securities, at fair Value

$ 443,884

$ 457,187

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References+ Details