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Derivative Instruments (Consolidated Balance Sheets And Fair Value Of The Principal Derivative Instruments) (Details) (USD $)
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Mar. 31, 2012
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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 | ||||||||||
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Other Assets [Member] | Interest Rate Futures [Member]
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| Derivative Assets [Abstract] | ||||||||||||
| Derivative Asset, Net | 613,000 | 612,000 | ||||||||||
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Other Assets [Member] | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member]
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| Derivative Assets [Abstract] | ||||||||||||
| Derivative Asset, Net | 9,406,000 | [1] | 0 | [1] | ||||||||
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Other Assets [Member] | Foreign Currency Forward Contracts, Investment Operations [Member]
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| Derivative Assets [Abstract] | ||||||||||||
| Derivative Asset, Net | 3,081,000 | [2] | 7,219,000 | [2] | ||||||||
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Other Assets [Member] | Foreign Currency Forward Contracts, Energy And Risk Operations [Member]
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| Derivative Assets [Abstract] | ||||||||||||
| Derivative Asset, Net | 140,000 | [3] | 387,000 | [3] | ||||||||
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Other Assets [Member] | Credit Default Swap [Member]
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| Derivative Assets [Abstract] | ||||||||||||
| Derivative Asset, Net | 568,000 | 0 | ||||||||||
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Other Assets [Member] | Energy And Weather Contracts [Member]
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| 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 | ||||||||||
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Other Liabilities [Member] | Interest Rate Futures [Member]
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| Derivative Liabilities [Abstract] | ||||||||||||
| Derivative Liabilities, Net | 202,000 | 339,000 | ||||||||||
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Other Liabilities [Member] | Foreign Currency Forward Contracts, Underwriting and Non-investment Operations [Member]
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| Derivative Liabilities [Abstract] | ||||||||||||
| Derivative Liabilities, Net | 6,322,000 | [1] | 11,754,000 | [1] | ||||||||
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Other Liabilities [Member] | Foreign Currency Forward Contracts, Investment Operations [Member]
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| Derivative Liabilities [Abstract] | ||||||||||||
| Derivative Liabilities, Net | 3,966,000 | [2] | 1,606,000 | [2] | ||||||||
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Other Liabilities [Member] | Credit Default Swap [Member]
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| Derivative Liabilities [Abstract] | ||||||||||||
| Derivative Liabilities, Net | 631,000 | 539,000 | ||||||||||
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Other Liabilities [Member] | Energy And Weather Contracts [Member]
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| 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 | ||||||||||
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