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	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	$ 26,750,242	[1]	$ 27,322,842	[1]
Gross unrealized gains of Fixed maturities	1,142,508		813,537
Related changes in estimated fair value of Fixed maturities	(689,436)		(765,447)
OTTI included in other comprehensive income (loss) of Fixed maturities	(197,964)	[2]	(228,827)	[2]
Fair value of Fixed maturities	27,005,350	[1]	27,142,105	[1]
Cost or Amortized cost of Short-term investments	367,878		450,491
Fair value of Short-term investments	366,679		450,681
Cost or Amortized cost of Equity securities	437,668	[3]	56,737
Gross unrealized gains of Equity securities	27,324	[3]	28,083
Related changes in estimated fair value of Equity securities	(55,829)	[3]	(53)
Fair value of Equity securities	409,163	[3],[4]	84,767	[4]
OTTI included in other comprehensive income (loss) of Equity securities		[2],[3]		[2]
Cost or Amortized cost of Total Investments - AFS	27,555,788		27,830,070
Gross unrealized gains of Total Investments - AFS	1,170,023		842,300
Related changes in estimated fair value of Total Investments - AFS	(746,655)		(765,990)
OTTI included in other comprehensive income (loss) of Total Investments - AFS	(197,964)	[2]	(228,827)	[2]
Fair value of Total Investments - AFS	27,781,192		27,677,553
Fixed maturities – HTM
Cost or Amortized cost of Equity securities	2,725,623		2,728,335
Gross unrealized gains of Equity securities	196,481		38,982
Related changes in estimated fair value of Equity securities	(20,670)		(24,691)
OTTI included in other comprehensive income (loss) of Equity securities		[2]		[2]
Fair value of Held to maturity securities	2,901,434		2,742,626
Short-term Investments [Member]
Fixed maturities – AFS
Cost or Amortized cost of Short-term investments	367,878	[5],[6]	450,491	[7],[8]
Gross unrealized gains of Short-term investments	191	[5],[6]	680	[7],[8]
Related changes in estimated fair value of Short-term investments	(1,390)	[5],[6]	(490)	[7],[8]
Fair value of Short-term investments	366,679	[5],[6]	450,681	[7],[8]
OTTI included in other comprehensive income (loss) of Short-term investments		[2],[5],[6]		[2],[7],[8]
US Government And Government Related [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	1,972,718	[6]	2,499,079	[8]
Gross unrealized gains of Fixed maturities	141,521	[6]	102,685	[8]
Related changes in estimated fair value of Fixed maturities	(5,398)	[6]	(36,320)	[8]
OTTI included in other comprehensive income (loss) of Fixed maturities		[2],[6]		[2],[8]
Fair value of Fixed maturities	2,108,841	[6]	2,565,444	[8]
Fixed maturities – HTM
Cost or Amortized cost of Equity securities	10,494	[6]	10,541	[8]
Gross unrealized gains of Equity securities	1,300	[6]	164	[8]
Related changes in estimated fair value of Equity securities		[6]	(9)	[8]
OTTI included in other comprehensive income (loss) of Equity securities		[2],[6]		[2],[8]
Fair value of Held to maturity securities	11,794	[6]	10,696	[8]
Corporate Debt Securities [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	10,622,568	[5],[9]	10,962,804	[10],[7]
Gross unrealized gains of Fixed maturities	505,910	[5],[9]	361,154	[10],[7]
Related changes in estimated fair value of Fixed maturities	(306,574)	[5],[9]	(281,849)	[10],[7]
OTTI included in other comprehensive income (loss) of Fixed maturities	(63,996)	[2],[5],[9]	(73,138)	[10],[2],[7]
Fair value of Fixed maturities	10,757,908	[5],[9]	10,968,971	[10],[7]
Fixed maturities – HTM
Cost or Amortized cost of Equity securities	1,318,854		1,337,797
Gross unrealized gains of Equity securities	54,862		6,370
Related changes in estimated fair value of Equity securities	(18,122)		(16,325)
OTTI included in other comprehensive income (loss) of Equity securities		[2]		[2]
Fair value of Held to maturity securities	1,355,594		1,327,842
Residential Mortgage-Backed Securities Agency [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	5,385,140		5,059,249
Gross unrealized gains of Fixed maturities	214,790		153,106
Related changes in estimated fair value of Fixed maturities	(4,791)		(8,644)
OTTI included in other comprehensive income (loss) of Fixed maturities		[2]		[2]
Fair value of Fixed maturities	5,595,139		5,203,711
Residential Mortgage-Backed Securities Non-Agency [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	889,949		1,257,474
Gross unrealized gains of Fixed maturities	19,689		26,361
Related changes in estimated fair value of Fixed maturities	(113,495)		(133,761)
OTTI included in other comprehensive income (loss) of Fixed maturities	(113,162)	[2]	(128,251)	[2]
Fair value of Fixed maturities	682,981		1,021,823
Fixed maturities – HTM
Cost or Amortized cost of Equity securities	81,923		82,763
Gross unrealized gains of Equity securities	4,898		634
Related changes in estimated fair value of Equity securities	(282)		(546)
OTTI included in other comprehensive income (loss) of Equity securities		[2]		[2]
Fair value of Held to maturity securities	86,539		82,851
Commercial Mortgage Backed Securities [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	978,131		1,135,075
Gross unrealized gains of Fixed maturities	50,171		55,852
Related changes in estimated fair value of Fixed maturities	(4,095)		(7,960)
OTTI included in other comprehensive income (loss) of Fixed maturities	(7,321)	[2]	(10,460)	[2]
Fair value of Fixed maturities	1,016,886		1,172,507
Collateralized Debt Obligations [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	871,407		920,501
Gross unrealized gains of Fixed maturities	7,380		11,014
Related changes in estimated fair value of Fixed maturities	(198,340)		(188,563)
OTTI included in other comprehensive income (loss) of Fixed maturities	(6,872)	[2]	(8,814)	[2]
Fair value of Fixed maturities	673,575		734,138
Other Asset Backed Securities [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	1,082,513		979,539
Gross unrealized gains of Fixed maturities	20,318		16,111
Related changes in estimated fair value of Fixed maturities	(19,393)		(26,954)
OTTI included in other comprehensive income (loss) of Fixed maturities	(6,613)	[2]	(8,164)	[2]
Fair value of Fixed maturities	1,076,825		960,532
Fixed maturities – HTM
Cost or Amortized cost of Equity securities	284,095		287,109
Gross unrealized gains of Equity securities	14,447		1,134
Related changes in estimated fair value of Equity securities	(16)		(1,410)
OTTI included in other comprehensive income (loss) of Equity securities		[2]		[2]
Fair value of Held to maturity securities	298,526		286,833
US States and Political Subdivisions Debt Securities [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	1,648,706		1,379,150
Gross unrealized gains of Fixed maturities	76,982		16,755
Related changes in estimated fair value of Fixed maturities	(2,134)		(35,449)
OTTI included in other comprehensive income (loss) of Fixed maturities		[2]		[2]
Fair value of Fixed maturities	1,723,554		1,360,456
Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities – AFS
Cost or Amortized cost of Fixed maturities	3,299,110	[6]	3,129,971	[8]
Gross unrealized gains of Fixed maturities	105,747	[6]	70,499	[8]
Related changes in estimated fair value of Fixed maturities	(35,216)	[6]	(45,947)	[8]
OTTI included in other comprehensive income (loss) of Fixed maturities		[2],[6]		[2],[8]
Fair value of Fixed maturities	3,369,641	[6]	3,154,523	[8]
Fixed maturities – HTM
Cost or Amortized cost of Equity securities	1,030,257	[6]	1,010,125	[8]
Gross unrealized gains of Equity securities	120,974	[6]	30,680	[8]
Related changes in estimated fair value of Equity securities	(2,250)	[6]	(6,401)	[8]
OTTI included in other comprehensive income (loss) of Equity securities		[2],[6]		[2],[8]
Fair value of Held to maturity securities	$ 1,148,981	[6]	$ 1,034,404	[8]

[1] Included in the table above are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are senior to the common and preferred equities of the financial institutions, at their fair value of $485.6 million and $757.8 million at September 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 $167.6 million and $143.7 million at September 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] Included within equity securities are investments in fixed income funds with a fair value of $91.1 million and an amortized cost of $100.0 million at September 30, 2011.

[4] Included within equity securities are investments fixed income funds of $91.1 million and nil at September 30, 2011 and December 31, 2010, respectively.

[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 $429.2 million and an amortized cost of $460.0 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] U.S. Government and Government-Related/Supported, Non-U.S. Sovereign Government, Provincials, Supranationals and Government-Related/Supported and Total short-term investments includes government-related securities with an amortized cost of $2,022.7 million and fair value of $2,058.5 million and U.S. Agencies with an amortized cost of $531.0 million and fair value of $581.5 million.

[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] U.S. Government and Government-Related/Supported, Non-U.S. Sovereign Government, Provincials, Supranationals and Government-Related/Supported and Total short-term investments 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.

[9] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are senior to the common and preferred equities of the financial institutions. These securities have a fair value of $485.6 million and an amortized cost of $653.2 million at September 30, 2011.

[10] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are 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.