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Invesments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $) | 3 Months Ended
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Mar. 31, 2012 | Dec. 31, 2011 | Mar. 31, 2011 | Dec. 31, 2010
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment income |  | $ 36,100,000 |  | $ 154,200,000
Realized gains (losses) |  | 750,700,000 |  | (192,300,000)
Net change in unrealized (depreciation) appreciation |  | (487,400,000) |  | 1,464,200,000
Net income |  | 299,400,000 |  | 1,426,100,000
Net investment income earned | 32,628,000 |  | 43,473,000 |
Alternative Investments [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment income earned | $ 2,000,000 |  | $ 11,600,000 |

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