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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $) | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012 | Mar. 31, 2013 Alternative Investments [Member] | Mar. 31, 2012 Alternative Investments [Member] | Dec. 31, 2012 Partnerships accounted for under the equity method [Member] | Dec. 31, 2011 Partnerships accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Net investment income |  |  |  |  | $ 204,800,000 | $ 36,100,000
Realized gains |  |  |  |  | 593,400,000 | 750,700,000
Net change in unrealized depreciation |  |  |  |  | (417,500,000) | (487,400,000)
Net income |  |  |  |  | 380,700,000 | 299,400,000
Net investment income earned | $ 32,870,000 | $ 32,628,000 | $ 3,600,000 | $ 2,000,000 |  |

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