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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $) | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2012 | Mar. 31, 2012 | Jun. 30, 2011 | Mar. 31, 2011 | Jun. 30, 2012 | Mar. 31, 2012 | Jun. 30, 2011 | Mar. 31, 2011
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income |  | $ 54,000,000 |  | $ 132,600,000 |  | $ 90,100,000 |  | $ 286,800,000
Realized gains (losses) |  | 234,600,000 |  | 355,300,000 |  | 985,300,000 |  | 163,000,000
Net change in unrealized (depreciation) appreciation |  | 53,400,000 |  | 608,300,000 |  | (434,000,000) |  | 2,072,500,000
Net income |  | 342,000,000 |  | 1,096,200,000 |  | 641,400,000 |  | 2,522,300,000
Net investment income earned | 34,006,000 |  | 39,345,000 |  | 66,634,000 |  | 82,818,000 |
Alternative Investments [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income earned |  | $ 3,000,000 |  | $ 7,900,000 |  | $ 5,000,000 |  | $ 19,500,000

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