Fair Value Measurements (Detail) - Schedule of assets and liabilities that were accounted for at fair value (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Assets
Fixed maturities, at fair value	$ 25,151,686	[1]	$ 25,544,179	[1]
Equity securities, at fair value	318,695		84,767
Short-term investments, at fair value	2,851,823	[2],[3]	2,048,607
Total investments available for sale	28,322,204		27,677,553
Cash equivalents (3)	1,642,540	[4]	1,899,265
Other investments (4)	642,270	[5]	624,037	[4]
Other assets (5)(6)	72,737	[6],[7]	92,943
Total assets accounted for at fair value	30,679,751		30,293,798	[6],[7]
Liabilities
Financial instruments sold, but not yet purchased (7)	26,149		21,526
Other liabilities (5)(6)	749,210		835,590
Total liabilities accounted for at fair value	111,994	[6],[7]	82,194	[6],[7]
Fair Value, Inputs, Level 1 [Member]
Assets
Equity securities, at fair value	244,798		71,284
Short-term investments, at fair value		[2],[3]
Total investments available for sale	244,798		71,284
Cash equivalents (3)	1,030,924	[4]	1,358,619
Other investments (4)		[5]		[4]
Other assets (5)(6)		[6],[7]
Total assets accounted for at fair value	1,275,722		1,429,903	[6],[7]
Liabilities
Financial instruments sold, but not yet purchased (7)			256
Total liabilities accounted for at fair value		[6],[7]	256	[6],[7]
Fair Value, Inputs, Level 1 [Member] | Fair Value Other Liabilities [Member]
Liabilities
Other liabilities (5)(6)		[8]		[8]
Fair Value, Inputs, Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets
Fixed maturities, at fair value		[2]
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Assets
Fixed maturities, at fair value		[2]
Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	24,390,224		24,722,765
Equity securities, at fair value	73,897		13,483
Short-term investments, at fair value	2,841,070	[2],[3]	2,046,424
Total investments available for sale	27,305,191		26,782,672
Cash equivalents (3)	611,616	[4]	540,646
Other investments (4)	527,730	[5]	490,320	[4]
Other assets (5)(6)	109,270	[6],[7]	108,056
Total assets accounted for at fair value	28,553,807		27,921,694	[6],[7]
Liabilities
Financial instruments sold, but not yet purchased (7)	26,149		21,270
Total liabilities accounted for at fair value	65,951	[6],[7]	34,861	[6],[7]
Fair Value, Inputs, Level 2 [Member] | Fair Value Other Liabilities [Member]
Liabilities
Other liabilities (5)(6)	39,802	[8]	13,591	[8]
Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Assets
Fixed maturities, at fair value	1,141,839		1,170,884
Fair Value, Inputs, Level 2 [Member] | Collateralized Debt Obligations [Member]
Assets
Fixed maturities, at fair value	8,718		12,566
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets
Fixed maturities, at fair value	1,518,812		2,127,491
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Fixed maturities, at fair value	10,264,823	[2]	10,325,725
Fair Value, Inputs, Level 2 [Member] | Residential Mortgage Backed Securities Agency [Member]
Assets
Fixed maturities, at fair value	5,660,955		5,134,491
Fair Value, Inputs, Level 2 [Member] | Residential Mortgage Backed Securities Non Agency [Member]
Assets
Fixed maturities, at fair value	770,092		1,016,124
Fair Value, Inputs, Level 2 [Member] | Other Asset Backed Securities [Member]
Assets
Fixed maturities, at fair value	977,460		924,181
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets
Fixed maturities, at fair value	1,463,803		1,351,677
Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Assets
Fixed maturities, at fair value	2,583,722		2,659,626
Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	761,462		821,414
Short-term investments, at fair value	10,753	[2],[3]	2,183
Total investments available for sale	772,215		823,597
Cash equivalents (3)		[4]
Other investments (4)	114,540	[5]	133,717	[4]
Other assets (5)(6)	409	[6],[7]	7,882
Total assets accounted for at fair value	887,164		965,196	[6],[7]
Liabilities
Total liabilities accounted for at fair value	50,054	[6],[7]	47,077	[6],[7]
Fair Value, Inputs, Level 3 [Member] | Fair Value Other Liabilities [Member]
Liabilities
Other liabilities (5)(6)	50,054	[8]	47,077	[8]
Fair Value, Inputs, Level 3 [Member] | Commercial Mortgage Backed Securities [Member]
Assets
Fixed maturities, at fair value	4,563		1,623
Fair Value, Inputs, Level 3 [Member] | Collateralized Debt Obligations [Member]
Assets
Fixed maturities, at fair value	727,239		721,097
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Assets
Fixed maturities, at fair value	4,768	[2]	35,158
Fair Value, Inputs, Level 3 [Member] | Residential Mortgage Backed Securities Agency [Member]
Assets
Fixed maturities, at fair value	15,385		30,255
Fair Value, Inputs, Level 3 [Member] | Residential Mortgage Backed Securities Non Agency [Member]
Assets
Fixed maturities, at fair value	3,194		4,964
Fair Value, Inputs, Level 3 [Member] | Other Asset Backed Securities [Member]
Assets
Fixed maturities, at fair value	6,313		24,650
Fair Value, Inputs, Level 3 [Member] | Foreign Government Debt Securities [Member]
Assets
Fixed maturities, at fair value			3,667
Collateral And Counterparty Netting [Member]
Assets
Short-term investments, at fair value		[2],[3]
Cash equivalents (3)		[4]
Other investments (4)		[5]		[4]
Other assets (5)(6)	(36,942)	[6],[7]	(22,995)
Total assets accounted for at fair value	(36,942)		(22,995)	[6],[7]
Liabilities
Total liabilities accounted for at fair value	(4,011)	[6],[7]		[6],[7]
Collateral And Counterparty Netting [Member] | Fair Value Other Liabilities [Member]
Liabilities
Other liabilities (5)(6)	(4,011)	[8]		[8]
Collateral And Counterparty Netting [Member] | Corporate Debt Securities [Member]
Assets
Fixed maturities, at fair value		[2]
Fair Value Other Liabilities [Member]
Liabilities
Other liabilities (5)(6)	85,845	[8]	60,668	[8]
Commercial Mortgage Backed Securities [Member]
Assets
Fixed maturities, at fair value	1,146,402	[1]	1,172,507	[1]
Collateralized Debt Obligations [Member]
Assets
Fixed maturities, at fair value	735,957	[1]	733,663	[1]
US Government-sponsored Enterprises Debt Securities [Member]
Assets
Fixed maturities, at fair value	1,518,812		2,127,491
Corporate Debt Securities [Member]
Assets
Fixed maturities, at fair value	10,269,591	[2]	10,360,883
Residential Mortgage Backed Securities Agency [Member]
Assets
Fixed maturities, at fair value	5,676,340		5,164,746
Residential Mortgage Backed Securities Non Agency [Member]
Assets
Fixed maturities, at fair value	773,286		1,021,088
Other Asset Backed Securities [Member]
Assets
Fixed maturities, at fair value	983,773		948,831
US States and Political Subdivisions Debt Securities [Member]
Assets
Fixed maturities, at fair value	1,463,803		1,351,677
Foreign Government Debt Securities [Member]
Assets
Fixed maturities, at fair value	$ 2,583,722		$ 2,663,293

[1] Included in the table above 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, at their fair value of $641.9 million and $757.8 million at June 30, 2011 and December 31, 2010, respectively. These securities are reflected in the table based on their call date and have net unrealized losses of $104.2 million and $143.7 million at June 30, 2011 and December 31, 2010, respectively.

[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, had a fair value of $491.3 million and $454.8 million and an amortized cost of $503.9 million and $504.6 million at June 30, 2011 and December 31, 2010, respectively. 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] Short-term investments consist primarily of Corporate, U.S. Government and Government-Related/Supported securities and Non-U.S. Sovereign Government, Supranational and Government-Related securities.

[4] Cash equivalents balances subject to fair value measurement include certificates of deposit and money market funds. Operating cash balances are not subject to fair value measurement guidance.

[5] The Other investments balance excludes certain structured transactions including certain investments in project finance transactions, a payment obligation and liquidity financing provided to a structured credit vehicle as a part of a third party medium term note facility. These investments are carried at amortized cost that totaled $325.9 million at June 30, 2011 and $327.7 million at December 31, 2010.

[6] Other assets and other liabilities include derivative instruments.

[7] The derivative balances included in each category above are reported on a gross basis by level with a netting adjustment presented separately in the "Collateral and Counterparty Netting" column. The Company often enters into different types of derivative contracts with a single counterparty and these contracts are covered under a netting agreement. In addition, the Company held net cash collateral related to derivative positions of approximately $32.9 million and $23.0 million at June 30, 2011 and December 31, 2010, respectively. This balance is included within cash and cash equivalents and the corresponding liability to return the collateral has been offset against the derivative positions within the balance sheet as appropriate under the netting agreement. The fair value of the individual derivative contracts are reported gross in their respective levels based on the fair value hierarchy.

[8] Financial instruments sold, but not yet purchased represent "short sales" and are included within "Net payable for investments purchased" on the balance sheet.