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Derivative Instruments (Consolidated Balance Sheets And Fair Value Of The Principal Derivative Instruments) (Details) (USD $) | Jun. 30, 2012 | Dec. 31, 2011
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Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | $ 37,945,000 |  | $ 60,939,000 |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 23,193,000 |  | 57,627,000 |
Other Assets [Member] | Interest Rate Futures [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 2,301,000 |  | 612,000 |
Other Assets [Member] | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 4,278,000 | [1] | 0 | [1]
Other Assets [Member] | Foreign Currency Forward Contracts, Energy And Risk Operations [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 0 | [2] | 387,000 | [2]
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 175,000 | [2] | 0 | [2]
Other Assets [Member] | Credit Default Swap [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 491,000 |  | 0 |
Other Assets [Member] | Energy And Weather Contracts [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 27,516,000 | [3] | 52,721,000 | [3]
Energy and Weather Contracts [Abstract] |  |  |  |
Derivative asset, gross asset component | 56,800,000 |  | 104,600,000 |
Derivative asset, gross liability component | 29,300,000 |  | 51,900,000 |
Other Liabilities [Member] | Interest Rate Futures [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 289,000 |  | 339,000 |
Other Liabilities [Member] | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 4,987,000 | [1] | 11,754,000 | [1]
Other Liabilities [Member] | Foreign Currency Forward Contracts, Investment Operations [Member] |  |  |  |
Derivative Assets [Abstract] |  |  |  |
Derivative Asset, Net | 3,359,000 | [4] | 7,219,000 | [4]
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 3,684,000 | [4] | 1,606,000 | [4]
Other Liabilities [Member] | Credit Default Swap [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 536,000 |  | 539,000 |
Other Liabilities [Member] | Energy And Weather Contracts [Member] |  |  |  |
Derivative Liabilities [Abstract] |  |  |  |
Derivative Liabilities, Net | 13,522,000 | [3] | 43,389,000 | [3]
Energy and Weather Contracts [Abstract] |  |  |  |
Derivative liability, gross asset component | 5,200,000 |  | 8,800,000 |
Derivative liability, gross liability component | $ 18,700,000 |  | $ 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 energy and risk operations.
[3] | Included in other assets is $56.8 million of derivative assets (December 31, 2011 – $104.6 million) and $29.3 million of derivative liabilities (December 31, 2011 – $51.9 million). Included in other liabilities is $5.2 million of derivative assets (December 31, 2011 – $8.8 million) and $18.7 million of derivative liabilities (December 31, 2011 – $52.2 million).
[4] | Contracts used to manage foreign currency risks in investment operations. Included in other liabilities are derivative assets of $3.4 million (December 31, 2011 – $7.2 million) which are netted with derivative liabilities of $3.7 million (December 31, 2011 – $1.6 million) under a master netting arrangements.

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