Investments (Detail) - Analysis of continual unrealized loss position (USD $) In Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2011		Dec. 31, 2010
Fixed maturities and short-term investments:
Total equity securities	$ 819,500
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	5,566,044		4,265,457
Gross Unrealized Losses of Fixed maturities and short term investments	(165,175)	[1]	(154,359)	[1]
Total equity securities	10,343		158
Total equity securities	(221)	[1]	(53)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	554,520		734,893
Gross Unrealized Losses of Fixed maturities and short term investments	(10,783)	[1]	(30,033)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member] | NonUS Sovereign Government Supranational And Government Related [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	822,154		459,686
Gross Unrealized Losses of Fixed maturities and short term investments	(7,510)	[1]	(5,116)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Commercial Mortgage Backed Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	106,076		46,419
Gross Unrealized Losses of Fixed maturities and short term investments	(3,694)	[1]	(2,472)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | US Government And Government Related [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	418,953		307,082
Gross Unrealized Losses of Fixed maturities and short term investments	(16,616)	[1]	(25,482)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Corporate Debt Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	2,429,908	[2],[3]	2,271,887	[4],[5]
Gross Unrealized Losses of Fixed maturities and short term investments	(86,771)	[1],[2],[3]	(80,276)	[1],[4],[5]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Residential Mortgage Backed Securities Agency [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	948,527		280,390
Gross Unrealized Losses of Fixed maturities and short term investments	(8,417)	[1]	(6,736)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Collateralized Debt Obligations [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	3,597		2,500
Gross Unrealized Losses of Fixed maturities and short term investments	(2,239)	[1]	(51)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Residential Mortgage Backed Securities Non Agency [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	98,770		40,052
Gross Unrealized Losses of Fixed maturities and short term investments	(27,038)	[1]	(2,574)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Other Asset Backed Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	183,539		122,548
Gross Unrealized Losses of Fixed maturities and short term investments	(2,107)	[1]	(1,619)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	3,455,090		4,092,776
Gross Unrealized Losses of Fixed maturities and short term investments	(654,131)	[1]	(840,405)	[1]
Total equity securities				[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	50,105		40,907
Gross Unrealized Losses of Fixed maturities and short term investments	(3,337)	[1]	(5,452)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member] | NonUS Sovereign Government Supranational And Government Related [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	368,376		418,322
Gross Unrealized Losses of Fixed maturities and short term investments	(33,052)	[1]	(40,837)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Commercial Mortgage Backed Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	43,771		69,475
Gross Unrealized Losses of Fixed maturities and short term investments	(9,328)	[1]	(15,967)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Collateralized Debt Obligations [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	713,897		715,295
Gross Unrealized Losses of Fixed maturities and short term investments	(157,429)	[1]	(197,535)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | US Government And Government Related [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	126,530		117,394
Gross Unrealized Losses of Fixed maturities and short term investments	(17,873)	[1]	(10,417)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Corporate Debt Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	1,374,319	[2],[3]	1,627,083	[4],[5]
Gross Unrealized Losses of Fixed maturities and short term investments	(214,481)	[1],[2],[3]	(275,023)	[1],[4],[5]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Residential Mortgage Backed Securities Agency [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	13,606		34,186
Gross Unrealized Losses of Fixed maturities and short term investments	(3,099)	[1]	(1,913)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Residential Mortgage Backed Securities Non Agency [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	573,274		843,168
Gross Unrealized Losses of Fixed maturities and short term investments	(193,306)	[1]	(259,715)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Other Asset Backed Securities [Member]
Fixed maturities and short-term investments:
Fair Value of Fixed maturities and short term investments	191,212		226,946
Gross Unrealized Losses of Fixed maturities and short term investments	$ (22,226)	[1]	$ (33,546)	[1]

[1] On securities impacted by the April 1, 2009 changes to OTTI values, length of time of impairment is measured from the point at which securities returned to a net unrealized loss position (i.e., from April 1, 2009).

[2] Included within Corporate are certain medium term notes supported primarily by pools of European credit with varying degrees of leverage. The notes, which are in a gross unrealized loss position, have a fair value of $491.3 million and an amortized cost of $503.9 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[3] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments senior to the common and preferred equities of the financial institutions. These securities, which are in a gross unrealized loss position, have a fair value of $641.9 million and an amortized cost of $746.1 million at June 30, 2011.

[4] Included within Corporate are certain medium term notes supported primarily by pools of European credit with varying degrees of leverage. The notes, which are in a gross unrealized loss position, have a fair value of $370.8 million and an amortized cost of $423.9 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[5] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments senior to the common and preferred equities of the financial institutions. These securities, which are in a gross unrealized loss position, have a fair value of $757.8 million and an amortized cost of $883.0 million at December 31, 2010.