Fixed Maturities And Equity Securities By Asset Class That Are Measured At Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	$ 2,045,963	$ 1,983,918
Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,943,563	1,888,069
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	418,876	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	398,399	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	385,239	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	20,254	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	49,077	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	212,609	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	667,179	683,976
Fixed maturities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	459,109	457,187
Equity Securities \| Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	102,400	95,849
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	284,101	232,474
Level 1 \| Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	181,701	136,625
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	181,701	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	102,400	95,849
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities and equity securities, at fair Value	1,761,862	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	1,761,862	1,751,444
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	237,175	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	398,399	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	385,239	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	20,254	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	49,077	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	212,609	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	667,179	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	$ 459,109	$ 457,187

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

Mar. 31, 2012

Dec. 31, 2011

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

Fixed maturities and equity securities, at fair Value

$ 2,045,963

$ 1,983,918

Fixed maturities

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

Fixed maturities and equity securities, at fair Value

1,943,563

1,888,069

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

418,876

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

398,399

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

385,239

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

20,254

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

49,077

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

212,609

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

667,179

683,976

Fixed maturities | Corporate bonds

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

Fixed maturities and equity securities, at fair Value

459,109

457,187

Equity Securities | Common stock

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

Fixed maturities and equity securities, at fair Value

102,400

95,849

Level 1

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

Fixed maturities and equity securities, at fair Value

284,101

232,474

Level 1 | Fixed maturities

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

Fixed maturities and equity securities, at fair Value

181,701

136,625

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

181,701

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

102,400

95,849

Level 2

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

Fixed maturities and equity securities, at fair Value

1,761,862

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

1,761,862

1,751,444

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

237,175

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

398,399

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

385,239

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

20,254

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

49,077

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

212,609

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

667,179

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

$ 459,109

$ 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