Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Assets
Fixed maturities, at fair value	$ 26,455,103		$ 27,846,920
Equity securities, at fair value	916,008		649,359
Short-term investments, at fair value	234,452		322,703
Total investments available for sale	27,605,563		28,818,982
U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	2,022,822	[1]	2,034,617	[1]
Corporate
Assets
Fixed maturities, at fair value	10,079,475	[2],[3]	10,482,000	[2],[3]
RMBS - Agency
Assets
Fixed maturities, at fair value	4,446,725		5,255,493
RMBS - Non-Agency
Assets
Fixed maturities, at fair value	406,402		647,733
CMBS
Assets
Fixed maturities, at fair value	1,129,072		1,077,251
CDO
Assets
Fixed maturities, at fair value	714,248		709,816
Other asset-backed securities
Assets
Fixed maturities, at fair value	1,188,760	[4]	1,470,711	[4]
U.S. States and political subdivisions of the States
Assets
Fixed maturities, at fair value	1,798,267		1,911,017
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	4,669,332	[1]	4,258,282	[1]
Recurring
Assets
Fixed maturities, at fair value	26,455,103		27,846,920
Equity securities, at fair value	916,008	[5]	649,359	[5]
Short-term investments, at fair value	234,452	[6],[7]	322,703	[6],[7]
Total investments available for sale	27,605,563		28,818,982
Cash equivalents	1,663,373	[8]	2,004,668	[8]
Other investments	842,387	[9]	907,755	[9]
Other assets	17,847	[10]	23,515	[10]
Total assets accounted for at fair value	30,129,170		31,754,920
Liabilities
Financial instruments sold, but not yet purchased	24,316	[11]	26,235	[11]
Other liabilities	37,321	[10]	65,299	[10]
Total liabilities accounted for at fair value	61,637		91,534
Recurring | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	2,022,822		2,034,617
Recurring | Corporate
Assets
Fixed maturities, at fair value	10,079,475	[12],[6]	10,482,000	[12],[6]
Recurring | RMBS - Agency
Assets
Fixed maturities, at fair value	4,446,725		5,255,493
Recurring | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	406,402		647,733
Recurring | CMBS
Assets
Fixed maturities, at fair value	1,129,072		1,077,251
Recurring | CDO
Assets
Fixed maturities, at fair value	714,248		709,816
Recurring | Other asset-backed securities
Assets
Fixed maturities, at fair value	1,188,760	[12]	1,470,711	[12]
Recurring | U.S. States and political subdivisions of the States
Assets
Fixed maturities, at fair value	1,798,267		1,911,017
Recurring | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	4,669,332		4,258,282
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, at fair value	0		0
Equity securities, at fair value	445,709	[5]	253,957	[5]
Short-term investments, at fair value	0	[6],[7]	0	[6],[7]
Total investments available for sale	445,709		253,957
Cash equivalents	1,375,683	[8]	1,596,376	[8]
Other investments			0
Other assets		[10]	0	[10]
Total assets accounted for at fair value	1,821,392		1,850,333
Liabilities
Financial instruments sold, but not yet purchased	375	[11]	0	[11]
Other liabilities		[10]	0	[10]
Total liabilities accounted for at fair value	375		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate
Assets
Fixed maturities, at fair value	0	[12],[6]	0	[12],[6]
Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS - Agency
Assets
Fixed maturities, at fair value	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | CMBS
Assets
Fixed maturities, at fair value	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | CDO
Assets
Fixed maturities, at fair value	0		0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Other asset-backed securities
Assets
Fixed maturities, at fair value	0	[12]	0	[12]
Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. States and political subdivisions of the States
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
Assets
Fixed maturities, at fair value	0		0
Recurring | Significant other observable inputs (Level 2)
Assets
Fixed maturities, at fair value	25,643,330		27,039,490
Equity securities, at fair value	470,299	[5]	395,402	[5]
Short-term investments, at fair value	232,453	[6],[7]	322,703	[6],[7]
Total investments available for sale	26,346,082		27,757,595
Cash equivalents	287,690	[8]	408,292	[8]
Other investments	738,543	[9]	792,483	[9]
Other assets	19,674	[10]	24,842	[10]
Total assets accounted for at fair value	27,391,989		28,983,212
Liabilities
Financial instruments sold, but not yet purchased	23,941	[11]	26,235	[11]
Other liabilities	9,774	[10]	30,379	[10]
Total liabilities accounted for at fair value	33,715		56,614
Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	2,022,822		2,034,617
Recurring | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value	10,047,302	[12],[6]	10,451,902	[12],[6]
Recurring | Significant other observable inputs (Level 2) | RMBS - Agency
Assets
Fixed maturities, at fair value	4,428,511		5,223,488
Recurring | Significant other observable inputs (Level 2) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	402,991		647,617
Recurring | Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities, at fair value	1,104,649		1,051,904
Recurring | Significant other observable inputs (Level 2) | CDO
Assets
Fixed maturities, at fair value	8,235		8,080
Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value	1,161,221	[12]	1,452,583	[12]
Recurring | Significant other observable inputs (Level 2) | U.S. States and political subdivisions of the States
Assets
Fixed maturities, at fair value	1,798,267		1,911,017
Recurring | Significant other observable inputs (Level 2) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	4,669,332		4,258,282
Recurring | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities, at fair value	811,773		807,430
Equity securities, at fair value	0	[5]	0	[5]
Short-term investments, at fair value	1,999	[6],[7]	0	[6],[7]
Total investments available for sale	813,772		807,430
Cash equivalents	0	[8]	0	[8]
Other investments	103,844	[9]	115,272	[9]
Other assets		[10]	0	[10]
Total assets accounted for at fair value	917,616		922,702
Liabilities
Financial instruments sold, but not yet purchased	0	[11]	0	[11]
Other liabilities	29,374	[10]	36,247	[10]
Total liabilities accounted for at fair value	29,374		36,247
Recurring | Significant other unobservable inputs (Level 3) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	0		0
Recurring | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities, at fair value	32,173	[12],[6]	30,098	[12],[6]
Recurring | Significant other unobservable inputs (Level 3) | RMBS - Agency
Assets
Fixed maturities, at fair value	18,214		32,005
Recurring | Significant other unobservable inputs (Level 3) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	3,411		116
Recurring | Significant other unobservable inputs (Level 3) | CMBS
Assets
Fixed maturities, at fair value	24,423		25,347
Recurring | Significant other unobservable inputs (Level 3) | CDO
Assets
Fixed maturities, at fair value	706,013		701,736
Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities
Assets
Fixed maturities, at fair value	27,539	[12]	18,128	[12]
Recurring | Significant other unobservable inputs (Level 3) | U.S. States and political subdivisions of the States
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
Assets
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting
Assets
Fixed maturities, at fair value	0		0
Equity securities, at fair value		[5]	0	[5]
Short-term investments, at fair value	0	[6],[7]	0	[6],[7]
Total investments available for sale	0		0
Cash equivalents	0	[8]	0	[8]
Other investments		[9]	0	[9]
Other assets	(1,827)	[10]	(1,327)	[10]
Total assets accounted for at fair value	(1,827)		(1,327)
Liabilities
Financial instruments sold, but not yet purchased		[11]	0	[11]
Other liabilities	(1,827)	[10]	(1,327)	[10]
Total liabilities accounted for at fair value	(1,827)		(1,327)
Recurring | Collateral and counterparty netting | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting | Corporate
Assets
Fixed maturities, at fair value	0	[12],[6]	0	[12],[6]
Recurring | Collateral and counterparty netting | RMBS - Agency
Assets
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting | CMBS
Assets
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting | CDO
Assets
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting | Other asset-backed securities
Assets
Fixed maturities, at fair value	0	[12]	0	[12]
Recurring | Collateral and counterparty netting | U.S. States and political subdivisions of the States
Assets
Fixed maturities, at fair value	0		0
Recurring | Collateral and counterparty netting | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	$ 0		$ 0

[1] 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 $2,357.0 million and fair value of $2,374.5 million and U.S. Agencies with an amortized cost of $297.2 million and fair value of $321.6 million.

[2] 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 $149.4 million and an amortized cost of $150.2 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.

[3] 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 $281.4 million and an amortized cost of $298.6 million at June 30, 2013.

[4] Covered Bonds within Fixed maturities - AFS with an amortized cost of $523.5 million and a fair value of $550.9 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.0 million and a fair value of $8.2 million are included within Other asset-backed securities to align the Company's classification to market indices.

[5] Included within Equity securities are investments in fixed income funds with a fair value of $91.3 million and $101.9 million at June 30, 2013 and December 31, 2012, respectively.

[6] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes had a fair value of $149.4 million and $194.3 million and an amortized cost of $150.2 million and $194.8 million at June 30, 2013 and December 31, 2012, 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.

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

[8] 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.

[9] 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 a part of a third party medium term note facility. These investments, which totaled $293.4 million at June 30, 2013 and $312.1 million at December 31, 2012, are carried at amortized cost. For further information, see Item 8, Note 7, “Other Investments,” to the Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2012.

[10] Other assets and other liabilities include derivative instruments. The derivative balances included in each category are reported on a gross basis by level with a netting adjustment presented separately in the Collateral and Counterparty Netting column. The fair values of the individual derivative contracts are reported gross in their respective levels based on the fair value hierarchy. For further details regarding derivative fair values and associated collateral received or paid see Note 6, “Derivative Instruments,” to the Unaudited Consolidated Financial Statements.

[11] Financial instruments sold, but not yet purchased, represent “short sales” and are included within “Payable for investments purchased” on the balance sheet.

[12] The Company invests in covered bonds (“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. At June 30, 2013 and December 31, 2012, Covered Bonds with a fair value of $550.9 million and $647.1 million, respectively, are included within Other asset-backed securities.