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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Jun. 30, 2012 | Sep. 30, 2011 | Jun. 30, 2011 | Sep. 30, 2012 | Jun. 30, 2012 | Sep. 30, 2011 | Jun. 30, 2011
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income |  | $ 80,000,000 |  | $ 136,800,000 |  | $ 170,100,000 |  | $ 423,600,000
Realized gains (losses) |  | 19,600,000 |  | 710,600,000 |  | 1,004,900,000 |  | 873,600,000
Net change in unrealized (depreciation) appreciation |  | (117,100,000) |  | (194,700,000) |  | (551,100,000) |  | 1,877,800,000
Net income |  | (17,500,000) |  | 652,700,000 |  | 623,900,000 |  | 3,175,000,000
Net investment income earned | 30,650,000 |  | 35,786,000 |  | 97,284,000 |  | 118,604,000 |
Alternative Investments [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income earned |  | $ 500,000 |  | $ 4,500,000 |  | $ 5,500,000 |  | $ 24,000,000

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