Investments (Detail) - Cost (amortized cost for fixed maturities and short-term investments), fair value, gross unrealized gains, gross unrealized (losses), and OTTI recorded in AOCI (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Fixed maturities
Cost or Amortized cost of Fixed maturities	$ 25,105,488	[1]	$ 25,714,886	[1]
Gross unrealized gains of Fixed maturities	826,736		794,611
Related changes in estimated fair value of Fixed maturities	(578,096)		(736,491)
OTTI included in other comprehensive income (loss) of Fixed maturities	(202,442)	[2]	(228,827)	[2]
Fair value of Fixed maturities	25,151,686	[1]	25,544,179	[1]
Cost or Amortized cost of Short-term investments	2,855,542		2,058,447
Fair value of Short-term investments	2,851,823		2,048,607
Cost or Amortized cost of Equity securities	282,128		56,737
Gross unrealized gains of Equity securities	36,788		28,083
Related changes in estimated fair value of Equity securities	(221)		(53)
Fair value of Equity securities	318,695		84,767
OTTI included in other comprehensive income (loss) of Equity securities		[2]		[2]
US Government And Government Related [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	1,454,860	[3]	2,052,551	[4]
Gross unrealized gains of Fixed maturities	79,258	[3]	98,889	[4]
Related changes in estimated fair value of Fixed maturities	(15,306)	[3]	(23,949)	[4]
OTTI included in other comprehensive income (loss) of Fixed maturities		[2],[3]		[2],[4]
Fair value of Fixed maturities	1,518,812	[3]	2,127,491	[4]
Corporate Debt Securities [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	10,196,731	[5],[6]	10,352,806	[7],[8]
Gross unrealized gains of Fixed maturities	366,054	[5],[6]	353,308	[7],[8]
Related changes in estimated fair value of Fixed maturities	(223,974)	[5],[6]	(272,093)	[7],[8]
OTTI included in other comprehensive income (loss) of Fixed maturities	(69,220)	[2],[5],[6]	(73,138)	[2],[7],[8]
Fair value of Fixed maturities	10,269,591	[5],[6]	10,360,883	[7],[8]
Residential Mortgage Backed Securities Agency [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	5,506,311		5,020,469
Gross unrealized gains of Fixed maturities	181,501		152,905
Related changes in estimated fair value of Fixed maturities	(11,472)		(8,628)
OTTI included in other comprehensive income (loss) of Fixed maturities		[2]		[2]
Fair value of Fixed maturities	5,676,340		5,164,746
Residential Mortgage Backed Securities Non Agency [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	970,742		1,256,741
Gross unrealized gains of Fixed maturities	22,885		26,356
Related changes in estimated fair value of Fixed maturities	(108,719)		(133,758)
OTTI included in other comprehensive income (loss) of Fixed maturities	(111,622)	[2]	(128,251)	[2]
Fair value of Fixed maturities	773,286		1,021,088
Commercial Mortgage Backed Securities [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	1,097,687		1,135,075
Gross unrealized gains of Fixed maturities	61,737		55,852
Related changes in estimated fair value of Fixed maturities	(5,529)		(7,960)
OTTI included in other comprehensive income (loss) of Fixed maturities	(7,493)	[2]	(10,460)	[2]
Fair value of Fixed maturities	1,146,402		1,172,507
Collateralized Debt Obligations [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	884,809		920,080
Gross unrealized gains of Fixed maturities	10,816		10,960
Related changes in estimated fair value of Fixed maturities	(152,647)		(188,563)
OTTI included in other comprehensive income (loss) of Fixed maturities	(7,021)	[2]	(8,814)	[2]
Fair value of Fixed maturities	735,957		733,663
Other Asset Backed Securities [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	992,550		964,129
Gross unrealized gains of Fixed maturities	13,341		16,084
Related changes in estimated fair value of Fixed maturities	(15,032)		(23,218)
OTTI included in other comprehensive income (loss) of Fixed maturities	(7,086)	[2]	(8,164)	[2]
Fair value of Fixed maturities	983,773		948,831
US States and Political Subdivisions Debt Securities [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	1,445,473		1,370,378
Gross unrealized gains of Fixed maturities	32,450		16,746
Related changes in estimated fair value of Fixed maturities	(14,120)		(35,447)
OTTI included in other comprehensive income (loss) of Fixed maturities		[2]		[2]
Fair value of Fixed maturities	1,463,803		1,351,677
NonUS Sovereign Government Supranational And Government Related [Member]
Fixed maturities
Cost or Amortized cost of Fixed maturities	2,556,325	[3]	2,642,657	[4]
Gross unrealized gains of Fixed maturities	58,694	[3]	63,511	[4]
Related changes in estimated fair value of Fixed maturities	(31,297)	[3]	(42,875)	[4]
OTTI included in other comprehensive income (loss) of Fixed maturities		[2],[3]		[2],[4]
Fair value of Fixed maturities	2,583,722	[3]	2,663,293	[4]
Short-term Investments [Member]
Fixed maturities
Cost or Amortized cost of Short-term investments	2,855,542	[5]	2,058,447	[7]
Gross unrealized gains of Short-term investments	35,049	[5]	19,606	[7]
Related changes in estimated fair value of Short-term investments	(38,768)	[5]	(29,446)	[7]
Fair value of Short-term investments	2,851,823	[5]	2,048,607	[7]
OTTI included in other comprehensive income (loss) of Short-term investments		[2],[5]		[2],[7]

[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] Represents the amount of OTTI losses in AOCI, which from April 1, 2009 was not included in earnings under authoritative accounting guidance.

[3] U.S. Government and Government-Related/Supported and Non-U.S. Sovereign Government, Supranationals and Government-Related/Supported includes government-related securities with an amortized cost of $2,014.9 million and fair value of $2,025.0 million and U.S. Agencies with an amortized cost of $699.6 million and fair value of $726.1 million.

[4] U.S. Government and Government-Related/Supported and Non-U.S. Sovereign Government, Supranationals and Government-Related/Supported includes government-related securities with an amortized cost of $2,101.0 million and fair value of $2,131.2 million and U.S. Agencies with an amortized cost of $1,019.2 million and fair value of $1,072.6 million.

[5] Included within Corporate are certain medium term notes supported primarily by pools of European credit with varying degrees of leverage. The notes 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.

[6] 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 have a fair value of $641.9 million and an amortized cost of $746.1 million at June 30, 2011.

[7] Included within Corporate are certain medium term notes supported primarily by pools of European credit with varying degrees of leverage. The notes have a fair value of $454.8 million and an amortized cost of $504.6 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.

[8] 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 have a fair value of $757.8 million and an amortized cost of $883.0 million at December 31, 2010.