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Derivative Instruments (Consolidated Balance Sheets And Fair Value Of The Principal Derivative Instruments) (Details) (USD $) | Mar. 31, 2012 | Dec. 31, 2011
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Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | $ 67,172,000 |  | $ 60,939,000 |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 33,767,000 |  | 57,627,000 |
Other Assets [Member] | Interest Rate Futures [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 613,000 |  | 612,000 |
Other Assets [Member] | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 9,406,000 | [1] | 0 | [1]
Other Assets [Member] | Foreign Currency Forward Contracts, Investment Operations [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 3,081,000 | [2] | 7,219,000 | [2]
Other Assets [Member] | Foreign Currency Forward Contracts, Energy And Risk Operations [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 140,000 | [3] | 387,000 | [3]
Other Assets [Member] | Credit Default Swap [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 568,000 |  | 0 |
Other Assets [Member] | Energy And Weather Contracts [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 53,364,000 | [4] | 52,721,000 | [4]
Energy and Weather Contracts [Abstract] |  |  |  |
Derivative asset, gross asset component | 171,200,000 |  | 104,600,000 |
Derivative asset, gross liability component | 117,800,000 |  | 51,900,000 |
Other Liabilities [Member] | Interest Rate Futures [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 202,000 |  | 339,000 |
Other Liabilities [Member] | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 6,322,000 | [1] | 11,754,000 | [1]
Other Liabilities [Member] | Foreign Currency Forward Contracts, Investment Operations [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 3,966,000 | [2] | 1,606,000 | [2]
Other Liabilities [Member] | Credit Default Swap [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 631,000 |  | 539,000 |
Other Liabilities [Member] | Energy And Weather Contracts [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 22,646,000 | [4] | 43,389,000 | [4]
Energy and Weather Contracts [Abstract] |  |  |  |
Derivative liability, gross asset component | 7,300,000 |  | 8,800,000 |
Derivative liability, gross liability component | $ 29,900,000 |  | $ 52,200,000 |
[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.
[4] | Included in other assets is $171.2 million of derivative assets (December 31, 2011 – $104.6 million) and $117.8 million of derivative liabilities (December 31, 2011 – $51.9 million). Included in other liabilities is $7.3 million of derivative assets (December 31, 2011 – $8.8 million) and $29.9 million of derivative liabilities (December 31, 2011 – $52.2 million).

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