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, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	$ 26,695,459	[1]	$ 25,771,715	[1]
Fair value of Fixed maturities	27,291,662	[1]	26,190,025	[1],[2]
Cost or Amortized cost of Short-term investments	212,393		359,378
Fair value of Short-term investments	212,327		359,063
Cost or Amortized cost of Equity securities	593,168		480,685
Fair value of Equity securities	631,068	[3]	468,197	[3]
Fair value of Total investments - AFS	28,135,057		27,017,285
Fixed maturities - HTM
Cost or Amortized cost of Fixed maturities - HTM	2,769,800		2,668,978
Gross unrealized gains of Fixed maturities - HTM	233,826		247,445
Related changes in estimated fair value of Fixed maturities - HTM	(9,197)		(20,735)
OTTI included in other comprehensive income (loss) of Fixed maturities - HTM	0		0
Fair value of Fixed maturities - HTM	2,994,429		2,895,688
US Government And Government Related [Member]
Fixed maturities - HTM
Cost or Amortized cost of Fixed maturities - HTM	10,683	[4]	10,399	[5]
Gross unrealized gains of Fixed maturities - HTM	1,174	[4]	1,510	[5]
Related changes in estimated fair value of Fixed maturities - HTM	0	[4]	0	[5]
OTTI included in other comprehensive income (loss) of Fixed maturities - HTM	0	[4]	0	[5]
Fair value of Fixed maturities - HTM	11,857	[4]	11,909	[5]
Corporate Debt Securities [Member]
Fixed maturities - HTM
Cost or Amortized cost of Fixed maturities - HTM	1,374,617	[6]	1,290,209	[5]
Gross unrealized gains of Fixed maturities - HTM	94,905	[6]	91,313	[5]
Related changes in estimated fair value of Fixed maturities - HTM	(4,164)	[6]	(14,433)	[5]
OTTI included in other comprehensive income (loss) of Fixed maturities - HTM	0	[6]	0	[5]
Fair value of Fixed maturities - HTM	1,465,358	[6]	1,367,089	[5]
Residential Mortgage-Backed Securities Non-Agency [Member]
Fixed maturities - HTM
Cost or Amortized cost of Fixed maturities - HTM	83,023		80,955
Gross unrealized gains of Fixed maturities - HTM	4,259		6,520
Related changes in estimated fair value of Fixed maturities - HTM	(147)		(32)
OTTI included in other comprehensive income (loss) of Fixed maturities - HTM	0		0
Fair value of Fixed maturities - HTM	87,135		87,443
Commercial Mortgage Backed Securities [Member]
Fixed maturities - HTM
Cost or Amortized cost of Fixed maturities - HTM	12,912
Gross unrealized gains of Fixed maturities - HTM	1,211
Related changes in estimated fair value of Fixed maturities - HTM	0
OTTI included in other comprehensive income (loss) of Fixed maturities - HTM	0
Fair value of Fixed maturities - HTM	14,123
Other Asset Backed Securities [Member]
Fixed maturities - HTM
Cost or Amortized cost of Fixed maturities - HTM	222,157	[6]	288,741	[5]
Gross unrealized gains of Fixed maturities - HTM	14,004	[6]	20,875	[5]
Related changes in estimated fair value of Fixed maturities - HTM	(573)	[6]	(320)	[5]
OTTI included in other comprehensive income (loss) of Fixed maturities - HTM	0	[6]	0	[5]
Fair value of Fixed maturities - HTM	235,588	[6]	309,296	[5]
Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities - HTM
Cost or Amortized cost of Fixed maturities - HTM	1,066,408	[4]	998,674	[7]
Gross unrealized gains of Fixed maturities - HTM	118,273	[4]	127,227	[7]
Related changes in estimated fair value of Fixed maturities - HTM	(4,313)	[4]	(5,950)	[7]
OTTI included in other comprehensive income (loss) of Fixed maturities - HTM	0	[4]	0	[7]
Fair value of Fixed maturities - HTM	1,180,368	[4]	1,119,951	[7]
US Government And Government Related [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	2,130,093	[4]	1,864,354	[7]
Gross unrealized gains of Fixed maturities	110,804	[4]	130,874	[7]
Related changes in estimated fair value of Fixed maturities	(7,077)	[4]	(4,245)	[7]
OTTI included in other comprehensive income (loss) of Fixed maturities	0	[4]	0	[7]
Fair value of Fixed maturities	2,233,820	[4]	1,990,983	[7]
Corporate Debt Securities [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	9,854,093	[6],[8],[9]	9,866,677	[10],[11],[5]
Gross unrealized gains of Fixed maturities	557,350	[6],[8],[9]	527,192	[10],[11],[5]
Related changes in estimated fair value of Fixed maturities	(148,569)	[6],[8],[9]	(233,581)	[10],[11],[5]
OTTI included in other comprehensive income (loss) of Fixed maturities	(42,882)	[6],[8],[9]	(51,666)	[10],[11],[5]
Fair value of Fixed maturities	10,219,992	[12],[2],[8],[9]	10,108,622	[10],[11],[12],[2],[5]
Residential Mortgage-Backed Securities Agency [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	5,238,075		5,189,473
Gross unrealized gains of Fixed maturities	163,999		193,782
Related changes in estimated fair value of Fixed maturities	(8,683)		(3,849)
OTTI included in other comprehensive income (loss) of Fixed maturities	0		0
Fair value of Fixed maturities	5,393,391		5,379,406
Residential Mortgage-Backed Securities Non-Agency [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	792,800		851,557
Gross unrealized gains of Fixed maturities	23,227		19,667
Related changes in estimated fair value of Fixed maturities	(71,292)		(112,867)
OTTI included in other comprehensive income (loss) of Fixed maturities	(112,681)		(116,542)
Fair value of Fixed maturities	632,054		641,815
Fair value of Total investments - AFS	535,200
Commercial Mortgage Backed Securities [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	882,762		927,684
Gross unrealized gains of Fixed maturities	66,416		56,704
Related changes in estimated fair value of Fixed maturities	(1,407)		(2,405)
OTTI included in other comprehensive income (loss) of Fixed maturities	(3,618)		(7,148)
Fair value of Fixed maturities	944,153		974,835
Collateralized Debt Obligations [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	798,313		843,553
Gross unrealized gains of Fixed maturities	8,262		6,624
Related changes in estimated fair value of Fixed maturities	(155,123)		(186,578)
OTTI included in other comprehensive income (loss) of Fixed maturities	(4,887)		(4,997)
Fair value of Fixed maturities	646,565		658,602
Other Asset Backed Securities [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	1,423,283	[6]	1,341,309	[5]
Gross unrealized gains of Fixed maturities	36,312	[6]	30,731	[5]
Related changes in estimated fair value of Fixed maturities	(23,474)	[6]	(25,486)	[5]
OTTI included in other comprehensive income (loss) of Fixed maturities	(9,979)	[6]	(6,305)	[5]
Fair value of Fixed maturities	1,426,142	[2]	1,340,249	[2],[5]
US States and Political Subdivisions Debt Securities [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	1,663,058		1,698,573
Gross unrealized gains of Fixed maturities	106,968		101,025
Related changes in estimated fair value of Fixed maturities	(1,844)		(2,220)
OTTI included in other comprehensive income (loss) of Fixed maturities	0		0
Fair value of Fixed maturities	1,768,182		1,797,378
Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	3,912,982	[4]	3,188,535	[7]
Gross unrealized gains of Fixed maturities	127,717	[4]	127,439	[7]
Related changes in estimated fair value of Fixed maturities	(13,336)	[4]	(17,839)	[7]
OTTI included in other comprehensive income (loss) of Fixed maturities	0	[4]	0	[7]
Fair value of Fixed maturities	4,027,363	[4]	3,298,135	[7]
Debt Securities [Member]
Fixed maturities - AFS
Cost or Amortized cost of Fixed maturities	26,695,459		25,771,715
Gross unrealized gains of Fixed maturities	1,201,055		1,194,038
Related changes in estimated fair value of Fixed maturities	(430,805)		(589,070)
OTTI included in other comprehensive income (loss) of Fixed maturities	(174,047)		(186,658)
Fair value of Fixed maturities	27,291,662		26,190,025
Equity Securities [Member]
Fixed maturities - AFS
Cost or Amortized cost of Equity securities	593,168		480,685
Gross unrealized gains of Equity securities	44,609		27,947
Related changes in estimated fair value of Equity securities	(6,709)		(40,435)
Fair value of Equity securities	631,068		468,197
OTTI included in other comprehensive income (loss) of Equity securities	0		0
Available-for-sale Securities [Member]
Fixed maturities - AFS
Cost or Amortized Cost of Total investments - AFS	27,501,020		26,611,778
Gross unrealized gains of Total investments - AFS	1,245,927		1,222,504
Related changes in estimated fair value of Total investments - AFS	(437,843)		(630,339)
OTTI included in other comprehensive income (loss) of Total investments - AFS	(174,047)		(186,658)
Fair value of Total investments - AFS	28,135,057		27,017,285
Short-term Investments [Member]
Fixed maturities - AFS
Cost or Amortized cost of Short-term investments	212,393	[4]	359,378	[7]
Gross unrealized gains of Short-term investments	263	[4]	519	[7]
Related changes in estimated fair value of Short-term investments	(329)	[4]	(834)	[7]
Fair value of Short-term investments	212,327	[4]	359,063	[7]
OTTI included in other comprehensive income (loss) of Short-term investments	$ 0	[4]	$ 0	[7]

[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 $411.2 million and $386.1 million at March 31, 2012 and December 31, 2011, respectively. These securities are reflected in the table based on their call date and have net unrealized losses of $78.1 million and $108.8 million at March 31, 2012 and December 31, 2011, respectively.

[2] The Company invests in covered bonds issued by financial institutions ("Covered Bonds"). Covered Bonds are senior secured debt instruments issued by financial institutions and backed by over-collateralized pools of public sector or mortgage loans. During the three months ended March 31, 2012, Covered Bonds within Total fixed maturities with a fair value of $415.5 million, have been included within Other asset-backed securities to align the Company's classification to market indices. At December 31, 2011 Covered Bonds within Total fixed maturities with a fair value of $353.9 million, have been reclassified from Corporate to Other asset-backed securities to conform to current period presentation.

[3] Included within equity securities are investments in fixed income funds of $98.0 million and $91.6 million at March 31, 2012 and December 31, 2011, respectively.

[4] 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 $2,272.1 million and fair value of $2,313.4 million and U.S. Agencies with an amortized cost of $461.2 million and fair value of $499.4 million.

[5] Covered Bonds within Fixed maturities - AFS with an amortized cost of $345.4 million and a fair value of $353.9 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.1 million and a fair value of $7.7 million at December 31, 2011 have been reclassified from Corporate to Other asset-backed securities to align the Company's classification to market indices and conform to current period presentation.

[6] During the three months ended March 31, 2012, Covered Bonds within Fixed maturities - AFS with an amortized cost of $398.6 million and a fair value of $415.5 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.3 million and a fair value of $7.7 million have been included within Other asset-backed securities to align the Company's classification to market indices. Covered Bonds were previously included in Corporate.

[7] 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 $1,878.3 million and fair value of $1,915.6 million and U.S. Agencies with an amortized cost of $494.0 million and fair value of $541.2 million.

[8] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $198.7 million and an amortized cost of $220.0 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.

[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 $411.2 million and an amortized cost of $489.3 million at March 31, 2012.

[10] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $266.0 million and an amortized cost of $297.7 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.

[11] 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 $386.1 million and an amortized cost of $494.9 million at December 31, 2011.

[12] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes, which are in a gross unrealized loss position, had a fair value of $198.7 million and $266.0 million and an amortized cost of $220.0 million and $297.7 million at March 31, 2012 and December 31, 2011, 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.