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Derivative Instruments (Gain (Loss) Recognized In The Consolidated Statements Of Operations Related To Its Derivative Instruments) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 0 Months Ended | 6 Months Ended
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Jun. 30, 2012 | Jun. 30, 2011 | Jun. 30, 2012 | Jun. 30, 2011 | Jun. 30, 2012 Net Investment Income [Member] Interest Rate Futures [Member] | Jun. 30, 2011 Net Investment Income [Member] Interest Rate Futures [Member] | Jun. 30, 2012 Net Investment Income [Member] Interest Rate Futures [Member] | Jun. 30, 2011 Net Investment Income [Member] Interest Rate Futures [Member] | Jun. 30, 2012 Net Investment Income [Member] Credit Default Swap [Member] | Jun. 30, 2011 Net Investment Income [Member] Credit Default Swap [Member] | Jun. 30, 2012 Net Investment Income [Member] Credit Default Swap [Member] | Jun. 30, 2011 Net Investment Income [Member] Credit Default Swap [Member] | Jun. 30, 2012 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] | Jun. 30, 2011 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] | Jun. 30, 2012 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] | Jun. 30, 2011 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] | Jun. 30, 2012 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Investment Operations [Member] | Jun. 30, 2011 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Investment Operations [Member] | Jun. 30, 2012 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Investment Operations [Member] | Jun. 30, 2011 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Investment Operations [Member] | Jun. 30, 2012 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Energy And Risk Operations [Member] | Jun. 30, 2011 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Energy And Risk Operations [Member] | Jun. 30, 2012 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Energy And Risk Operations [Member] | Jun. 30, 2011 Net Foreign Exchange Gains (Losses) [Member] Foreign Currency Forward Contracts, Energy And Risk Operations [Member] | Jun. 30, 2012 Other Income (Loss) [Member] Energy And Weather Contracts [Member] | Jun. 30, 2011 Other Income (Loss) [Member] Energy And Weather Contracts [Member] | Jun. 30, 2012 Other Income (Loss) [Member] Energy And Weather Contracts [Member] | Jun. 30, 2011 Other Income (Loss) [Member] Energy And Weather Contracts [Member] | Jan. 20, 2011 Other Income (Loss) [Member] Platinum Warrant [Member] | Jun. 30, 2012 Other Income (Loss) [Member] Platinum Warrant [Member] | Jun. 30, 2011 Other Income (Loss) [Member] Platinum Warrant [Member]
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of gain (loss) recognized on derivatives | $ 9,208 | $ (4,600) | $ (18,476) | $ 906 | $ (2,847) | $ (7,693) | $ (1,817) | $ (8,347) | $ (24) | $ 420 | $ 534 | $ 1,142 | $ (2,183) | [1] | $ 9,208 | [1] | $ 1,369 | [1] | $ 17,007 | [1] | $ 1,899 | [2] | $ (7,752) | [2] | $ (2,349) | [2] | $ (21,152) | [2] | $ 376 | [3] | $ (212) | [3] | $ 513 | [3] | $ (648) | [3] | $ 11,987 | $ 1,429 | $ (16,726) | $ 9,929 | $ 3,000 | $ 0 | $ 2,975
[1] | Contracts used to manage foreign currency risks in underwriting and non-investment operations.
[2] | Contracts used to manage foreign currency risks in investment operations.
[3] | Contracts used to manage foreign currency risks in energy and risk operations.

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- Definition The amount of net gains and losses recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 205G -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5618551-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624171-113959 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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