Fair Value Measurements (Details) - Schedule of assets and liabilities that were accounted for at fair value (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
ASSETS
Fixed maturities, at fair value	$ 26,242,802		$ 26,190,025
Equity securities, at fair value	529,056		468,197
Short-term investments, at fair value	227,380		359,063
Total investments available for sale	26,999,238		27,017,285
U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,101,625		1,990,983
Corporate [Member]
ASSETS
Fixed maturities, at fair value	9,721,153		10,108,622
Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,202,313		5,379,406
Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	584,543		641,815
Commercial Mortgage Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	904,792		974,835
Collateralized Debt Obligations [Member]
ASSETS
Fixed maturities, at fair value	650,018		658,602
Other Asset Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	1,407,738		1,340,249
U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,764,431		1,797,378
Non US Sovereign Government Supranational And Government Related [Member]
ASSETS
Fixed maturities, at fair value	3,906,189		3,298,135
Fair Value Measurements Recurring Member
ASSETS
Fixed maturities, at fair value	26,242,802		26,190,025	[1]
Equity securities, at fair value	529,056	[2]	468,197	[2]
Short-term investments, at fair value	227,380	[3],[4]	359,063	[3],[4]
Total investments available for sale	26,999,238		27,017,285
Cash equivalents	2,461,281	[5]	2,754,365	[5]
Other investments	894,203	[6]	661,557	[6]
Other assets	53,430	[7]	65,734	[7]
Total assets accounted for at fair value	30,408,152		30,498,941
Liabilities:
Financial instruments sold, but not yet purchased	28,198	[8]	20,844	[8]
Other liabilities	46,525	[7]	58,706	[7]
Total liabilities accounted for at fair value	74,723		79,550
Fair Value Measurements Recurring Member | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,101,625		1,990,983
Fair Value Measurements Recurring Member | Corporate [Member]
ASSETS
Fixed maturities, at fair value	9,721,153	[1],[3]	10,108,622	[1],[3]
Fair Value Measurements Recurring Member | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,202,313		5,379,406
Fair Value Measurements Recurring Member | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	584,543		641,815
Fair Value Measurements Recurring Member | Commercial Mortgage Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	904,792		974,835
Fair Value Measurements Recurring Member | Collateralized Debt Obligations [Member]
ASSETS
Fixed maturities, at fair value	650,018		658,602
Fair Value Measurements Recurring Member | Other Asset Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	1,407,738	[1]	1,340,249	[1]
Fair Value Measurements Recurring Member | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,764,431		1,797,378
Fair Value Measurements Recurring Member | Non US Sovereign Government Supranational And Government Related [Member]
ASSETS
Fixed maturities, at fair value	3,906,189		3,298,135
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1)
ASSETS
Fixed maturities, at fair value	0		0	[1]
Equity securities, at fair value	273,427	[2]	239,175	[2]
Short-term investments, at fair value	0	[3],[4]	0	[3],[4]
Total investments available for sale	273,427		239,175
Cash equivalents	1,750,569	[5]	1,686,101	[5]
Other investments	0	[6]	0	[6]
Other assets	0	[7]	0	[7]
Total assets accounted for at fair value	2,023,996		1,925,276
Liabilities:
Financial instruments sold, but not yet purchased	0	[8]	0	[8]
Other liabilities	0	[7]	0	[7]
Total liabilities accounted for at fair value	0		0
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | Corporate [Member]
ASSETS
Fixed maturities, at fair value	0	[1],[3]	0	[1],[3]
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | Commercial Mortgage Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | Collateralized Debt Obligations [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | Other Asset Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	0	[1]	0	[1]
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Quoted prices in active markets for identical assets (Level 1) | Non US Sovereign Government Supranational And Government Related [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2)
ASSETS
Fixed maturities, at fair value	25,487,680		25,466,763	[1]
Equity securities, at fair value	255,629	[2]	229,022	[2]
Short-term investments, at fair value	227,380	[3],[4]	359,063	[3],[4]
Total investments available for sale	25,970,689		26,054,848
Cash equivalents	710,712	[5]	1,068,264	[5]
Other investments	776,438	[6]	547,598	[6]
Other assets	178,104	[7]	143,622	[7]
Total assets accounted for at fair value	27,635,943		27,814,332
Liabilities:
Financial instruments sold, but not yet purchased	28,198	[8]	20,844	[8]
Other liabilities	10,578	[7]	16,871	[7]
Total liabilities accounted for at fair value	38,776		37,715
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,101,625		1,990,983
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | Corporate [Member]
ASSETS
Fixed maturities, at fair value	9,687,642	[1],[3]	10,084,804	[1],[3]
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,149,725		5,347,365
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	582,947		641,815
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | Commercial Mortgage Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	904,792		974,835
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | Collateralized Debt Obligations [Member]
ASSETS
Fixed maturities, at fair value	7,839		7,751
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | Other Asset Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	1,382,490	[1]	1,323,697	[1]
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,764,431		1,797,378
Fair Value Measurements Recurring Member | Significant other observable inputs (Level 2) | Non US Sovereign Government Supranational And Government Related [Member]
ASSETS
Fixed maturities, at fair value	3,906,189		3,298,135
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3)
ASSETS
Fixed maturities, at fair value	755,122		723,262	[1]
Equity securities, at fair value	0	[2]	0	[2]
Short-term investments, at fair value	0	[3],[4]	0	[3],[4]
Total investments available for sale	755,122		723,262
Cash equivalents	0	[5]	0	[5]
Other investments	117,765	[6]	113,959	[6]
Other assets	0	[7]	0	[7]
Total assets accounted for at fair value	872,887		837,221
Liabilities:
Financial instruments sold, but not yet purchased	0	[8]	0	[8]
Other liabilities	35,947	[7]	42,644	[7]
Total liabilities accounted for at fair value	35,947		42,644
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | Corporate [Member]
ASSETS
Fixed maturities, at fair value	33,511	[1],[3]	23,818	[1],[3]
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	52,588		32,041
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	1,596		0
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | Commercial Mortgage Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | Collateralized Debt Obligations [Member]
ASSETS
Fixed maturities, at fair value	642,179		650,851
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | Other Asset Backed Securities [Member]
ASSETS
Fixed maturities, at fair value	25,248	[1]	16,552	[1]
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Significant other unobservable inputs (Level 3) | Non US Sovereign Government Supranational And Government Related [Member]
ASSETS
Fixed maturities, at fair value	0		0
Fair Value Measurements Recurring Member | Collateral and counterparty netting
ASSETS
Other assets	(124,674)	[7]	(77,888)	[7]
Total assets accounted for at fair value	(124,674)		(77,888)
Liabilities:
Other liabilities	0	[7]	(809)	[7]
Total liabilities accounted for at fair value	$ 0		$ (809)

[1] 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. Covered Bonds are included within Other asset-backed securities to align the Company's classification to market indices.

[2] Certain investments in fixed income funds are included within equity securities.

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

[4] Short-term investments consist primarily of Corporate securities and U.S. Government and Government-Related/Supported securities.

[5] 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.

[6] 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 part of a third party medium term note facility. These investments are carried at amortized cost.

[7] Other assets and other liabilities include derivative instruments. 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 Counterpary 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.. Net cash collateral related to derivative positions 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 values 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 "Payable for investments purchased" on the balance sheet.