Fair Value Measurements (Details) - Schedule of assets and liabilities that were accounted for at fair value (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
ASSETS
Fixed maturities, at fair value	$ 27,426,293		$ 26,190,025
Equity securities, at fair value	565,927		468,197
Short-term investments, at fair value	323,705		359,063
Total investments available for sale	28,315,925		27,017,285
U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,135,368		1,990,983
Corporate [Member]
ASSETS
Fixed maturities, at fair value	10,007,137		10,108,622
Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,376,903		5,379,406
Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	636,325		641,815
Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	1,034,409		974,835
Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	682,566		658,602
Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	1,458,544		1,340,249
U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,820,105		1,797,378
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	4,274,936		3,298,135
Recurring
ASSETS
Fixed maturities, at fair value	27,426,293		26,190,025	[1]
Equity securities, at fair value	565,927	[2]	468,197	[2]
Short-term investments, at fair value	323,705	[3],[4]	359,063	[3],[4]
Total investments available for sale	28,315,925		27,017,285
Cash equivalents	2,359,617	[5]	2,754,365	[5]
Other investments	903,788	[6]	661,557	[6]
Other assets	52,151	[7]	65,734	[7]
Total assets accounted for at fair value	31,631,481		30,498,941
Liabilities:
Financial instruments sold, but not yet purchased	30,944	[8]	20,844	[8]
Other liabilities	117,071	[7]	58,706	[7]
Total liabilities accounted for at fair value	148,015		79,550
Recurring | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,135,368		1,990,983
Recurring | Corporate [Member]
ASSETS
Fixed maturities, at fair value	10,007,137	[1],[3]	10,108,622	[1],[3]
Recurring | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,376,903		5,379,406
Recurring | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	636,325		641,815
Recurring | Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	1,034,409		974,835
Recurring | Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	682,566		658,602
Recurring | Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	1,458,544	[1]	1,340,249	[1]
Recurring | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,820,105		1,797,378
Recurring | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	4,274,936		3,298,135
Recurring | Quoted prices in active markets for identical assets (Level 1)
ASSETS
Fixed maturities, at fair value	0		0	[1]
Equity securities, at fair value	230,539	[2]	239,175	[2]
Short-term investments, at fair value	0	[3],[4]	0	[3],[4]
Total investments available for sale	230,539		239,175
Cash equivalents	2,037,971	[5]	1,686,101	[5]
Other investments	0	[6]	0	[6]
Other assets	0	[7]	0	[7]
Total assets accounted for at fair value	2,268,510		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
Recurring | 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
Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate [Member]
ASSETS
Fixed maturities, at fair value	0	[1],[3]	0	[1],[3]
Recurring | 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
Recurring | 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
Recurring | Quoted prices in active markets for identical assets (Level 1) | Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	0		0
Recurring | 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]
Recurring | 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
Recurring | Quoted prices in active markets for identical assets (Level 1) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	0		0
Recurring | Significant other observable inputs (Level 2)
ASSETS
Fixed maturities, at fair value	26,630,137		25,466,763	[1]
Equity securities, at fair value	335,388	[2]	229,022	[2]
Short-term investments, at fair value	323,705	[3],[4]	359,063	[3],[4]
Total investments available for sale	27,289,230		26,054,848
Cash equivalents	321,646	[5]	1,068,264	[5]
Other investments	783,813	[6]	547,598	[6]
Other assets	10,611	[7]	143,622	[7]
Total assets accounted for at fair value	28,405,300		27,814,332
Liabilities:
Financial instruments sold, but not yet purchased	30,944	[8]	20,844	[8]
Other liabilities	82,009	[7]	16,871	[7]
Total liabilities accounted for at fair value	112,953		37,715
Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,135,368		1,990,983
Recurring | Significant other observable inputs (Level 2) | Corporate [Member]
ASSETS
Fixed maturities, at fair value	9,978,002	[1],[3]	10,084,804	[1],[3]
Recurring | Significant other observable inputs (Level 2) | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,343,046		5,347,365
Recurring | Significant other observable inputs (Level 2) | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	628,486		641,815
Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	1,001,040		974,835
Recurring | Significant other observable inputs (Level 2) | Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	7,773		7,751
Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	1,441,381	[1]	1,323,697	[1]
Recurring | Significant other observable inputs (Level 2) | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,820,105		1,797,378
Recurring | Significant other observable inputs (Level 2) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	4,274,936		3,298,135
Recurring | Significant other unobservable inputs (Level 3)
ASSETS
Fixed maturities, at fair value	796,156		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	796,156		723,262
Cash equivalents	0	[5]	0	[5]
Other investments	119,975	[6]	113,959	[6]
Other assets	0	[7]	0	[7]
Total assets accounted for at fair value	916,131		837,221
Liabilities:
Financial instruments sold, but not yet purchased	0	[8]	0	[8]
Other liabilities	35,062	[7]	42,644	[7]
Total liabilities accounted for at fair value	35,062		42,644
Recurring | Significant other unobservable inputs (Level 3) | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	0		0
Recurring | Significant other unobservable inputs (Level 3) | Corporate [Member]
ASSETS
Fixed maturities, at fair value	29,135	[1],[3]	23,818	[1],[3]
Recurring | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	33,857		32,041
Recurring | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	7,839		0
Recurring | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	33,369		0
Recurring | Significant other unobservable inputs (Level 3) | Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	674,793		650,851
Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	17,163	[1]	16,552	[1]
Recurring | Significant other unobservable inputs (Level 3) | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	0		0
Recurring | Significant other unobservable inputs (Level 3) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting
ASSETS
Other assets	41,540	[7]	(77,888)	[7]
Total assets accounted for at fair value	41,540		(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.