Schedule I. Summary of Investments Other Than Investments in Related Parties (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Market Value	$ 6,743,750
Amount at which shown in the Balance Sheet	6,743,750
Short-term investments	1,013,222us-gaap_OtherShortTermInvestments	1,044,779us-gaap_OtherShortTermInvestments
Equity investments trading	322,098us-gaap_TradingSecuritiesEquity	254,776us-gaap_TradingSecuritiesEquity
Other investments	504,147us-gaap_OtherInvestments	573,264us-gaap_OtherInvestments
Investments in other ventures, under equity method	120,713us-gaap_EquityMethodInvestments	105,616us-gaap_EquityMethodInvestments
U.S. treasuries
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,672,441
Market Value	1,671,471
Amount at which shown in the Balance Sheet	1,671,471
Agencies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	96,271
Market Value	96,208
Amount at which shown in the Balance Sheet	96,208
Non-U.S. government (Sovereign Debt)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	287,856
Market Value	280,651
Amount at which shown in the Balance Sheet	280,651
Non-U.S. government-backed corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	146,691
Market Value	146,467
Amount at which shown in the Balance Sheet	146,467
Corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,611,172
Market Value	1,610,442
Amount at which shown in the Balance Sheet	1,610,442
Agency mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	315,911
Market Value	316,620
Amount at which shown in the Balance Sheet	316,620
Non-agency mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	237,891
Market Value	253,050
Amount at which shown in the Balance Sheet	253,050
Commercial mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	377,792
Market Value	381,051
Amount at which shown in the Balance Sheet	381,051
Asset-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	27,360
Market Value	27,610
Amount at which shown in the Balance Sheet	27,610
Fixed maturity investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,773,385
Market Value	4,783,570
Amount at which shown in the Balance Sheet	$ 4,783,570

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Other investments not otherwise specified in the taxonomy.

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Other investments not otherwise specified in the taxonomy that will be realized in a short period of time, usually less than one year or the normal operating cycle, whichever is longer.

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This element represents the carrying (reported) amount of the entity's investments as of the balance sheet date. This amount may represent the individual investment's or group of similar investments' original cost, fair value, or such other amount at which the investment is shown in the entity's balance sheet.

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Fair value of investments in trading equity securities and other forms of trading securities that provide ownership interests.

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