Fair Value Measurements (Details) - Schedule of assets and liabilities that were accounted for at fair value (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
ASSETS
Fixed maturities, at fair value	$ 27,846,920		$ 26,190,025
Equity securities, at fair value	649,359		468,197
Short-term investments, at fair value	322,703		359,063
Total investments available for sale	28,818,982		27,017,285
U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,034,617		1,990,983
Corporate [Member]
ASSETS
Fixed maturities, at fair value	10,482,000		10,108,622
Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,255,493		5,379,406
Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	647,733		641,815
Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	1,077,251		974,835
Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	709,816		658,602
Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	1,470,711		1,340,249
U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,911,017		1,797,378
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	4,258,282		3,298,135
Recurring
ASSETS
Equity securities, at fair value	649,359	[1]	468,197	[1]
Short-term investments, at fair value	322,703	[2],[3]	359,063	[2],[3]
Total investments available for sale	28,818,982		27,017,285
Cash equivalents	2,004,668	[4]	2,754,365	[4]
Other investments	907,755	[5]	661,557	[5]
Other assets	29,967	[6]	65,734	[6]
Total assets accounted for at fair value	31,761,372		30,498,941
Liabilities:
Financial instruments sold, but not yet purchased	26,235		20,844
Other liabilities	59,623	[6]	58,706	[6]
Total liabilities accounted for at fair value	85,858		79,550
Recurring | Debt Securities [Member]
ASSETS
Fixed maturities, at fair value	27,846,920		26,190,025	[7]
Recurring | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,034,617		1,990,983
Recurring | Corporate [Member]
ASSETS
Fixed maturities, at fair value	10,482,000	[2],[7]	10,108,622	[2],[7]
Recurring | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,255,493		5,379,406
Recurring | Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
ASSETS
Fixed maturities, at fair value	647,733		641,815
Recurring | Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	1,077,251		974,835
Recurring | Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	709,816		658,602
Recurring | Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	1,470,711	[7]	1,340,249	[7]
Recurring | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,911,017		1,797,378
Recurring | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	4,258,282		3,298,135
Recurring | Quoted prices in active markets for identical assets (Level 1)
ASSETS
Equity securities, at fair value	253,957	[1]	239,175	[1]
Short-term investments, at fair value	0	[2],[3]	0	[2],[3]
Total investments available for sale	253,957		239,175
Cash equivalents	1,596,376	[4]	1,686,101	[4]
Other investments	0	[5]	0	[5]
Other assets	0	[6]	0	[6]
Total assets accounted for at fair value	1,850,333		1,925,276
Liabilities:
Financial instruments sold, but not yet purchased	0		0
Other liabilities	0	[6]	0	[6]
Total liabilities accounted for at fair value	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Debt Securities [Member]
ASSETS
Fixed maturities, at fair value	0		0	[7]
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	[2],[7]	0	[2],[7]
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	[7]	0	[7]
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
Equity securities, at fair value	395,402	[1]	229,022	[1]
Short-term investments, at fair value	322,703	[2],[3]	359,063	[2],[3]
Total investments available for sale	27,757,595		26,054,848
Cash equivalents	408,292	[4]	1,068,264	[4]
Other investments	792,483	[5]	547,598	[5]
Other assets	17,837	[6]	143,622	[6]
Total assets accounted for at fair value	28,976,207		27,814,332
Liabilities:
Financial instruments sold, but not yet purchased	26,235		20,844
Other liabilities	23,376	[6]	16,871	[6]
Total liabilities accounted for at fair value	49,611		37,715
Recurring | Significant other observable inputs (Level 2) | Debt Securities [Member]
ASSETS
Fixed maturities, at fair value	27,039,490		25,466,763	[7]
Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported [Member]
ASSETS
Fixed maturities, at fair value	2,034,617		1,990,983
Recurring | Significant other observable inputs (Level 2) | Corporate [Member]
ASSETS
Fixed maturities, at fair value	10,451,902	[2],[7]	10,084,804	[2],[7]
Recurring | Significant other observable inputs (Level 2) | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	5,223,488		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	647,617		641,815
Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	1,051,904		974,835
Recurring | Significant other observable inputs (Level 2) | Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	8,080		7,751
Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	1,452,583	[7]	1,323,697	[7]
Recurring | Significant other observable inputs (Level 2) | U.S. States and political subdivisions of the States [Member]
ASSETS
Fixed maturities, at fair value	1,911,017		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	4,258,282		3,298,135
Recurring | Significant other unobservable inputs (Level 3)
ASSETS
Equity securities, at fair value	0	[1]	0	[1]
Short-term investments, at fair value	0	[2],[3]	0	[2],[3]
Total investments available for sale	807,430		723,262
Cash equivalents	0	[4]	0	[4]
Other investments	115,272	[5]	113,959	[5]
Other assets	0	[6]	0	[6]
Total assets accounted for at fair value	922,702		837,221
Liabilities:
Financial instruments sold, but not yet purchased	0		0
Other liabilities	36,247	[6]	42,644	[6]
Total liabilities accounted for at fair value	36,247		42,644
Recurring | Significant other unobservable inputs (Level 3) | Debt Securities [Member]
ASSETS
Fixed maturities, at fair value	807,430		723,262	[7]
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	30,098	[2],[7]	23,818	[2],[7]
Recurring | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
ASSETS
Fixed maturities, at fair value	32,005		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	116		0
Recurring | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities ("CMBS") [Member]
ASSETS
Fixed maturities, at fair value	25,347		0
Recurring | Significant other unobservable inputs (Level 3) | Collateralized debt obligations ("CDO") [Member]
ASSETS
Fixed maturities, at fair value	701,736		650,851
Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities [Member]
ASSETS
Fixed maturities, at fair value	18,128	[7]	16,552	[7]
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	12,130	[6]	(77,888)	[6]
Total assets accounted for at fair value	12,130		(77,888)
Liabilities:
Other liabilities			(809)	[6]
Total liabilities accounted for at fair value			$ (809)

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

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

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

[4] 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.

[5] 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.

[6] 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.

[7] 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.