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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 | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Mar. 31, 2013 Alternative Investments [Member] | Mar. 31, 2012 Alternative Investments [Member] | Mar. 31, 2013 Alternative Investments [Member] | Mar. 31, 2012 Alternative Investments [Member] | Mar. 31, 2013 Partnerships accounted for under the equity method [Member] | Mar. 31, 2012 Partnerships accounted for under the equity method [Member] | Mar. 31, 2013 Partnerships accounted for under the equity method [Member] | Mar. 31, 2012 Partnerships accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  | $ 46,800,000 | $ 54,000,000 | $ 255,000,000 | $ 90,100,000
Realized (losses) gains |  |  |  |  |  |  |  |  | (22,100,000) | 234,600,000 | 599,700,000 | 985,300,000
Net change in unrealized depreciation |  |  |  |  |  |  |  |  | 378,800,000 | 53,400,000 | (18,900,000) | (434,000,000)
Net income |  |  |  |  |  |  |  |  | 403,500,000 | 342,000,000 | 835,800,000 | 641,400,000
Net investment income earned | $ 34,003,000 | $ 34,006,000 | $ 66,873,000 | $ 66,634,000 | $ 3,900,000 | $ 3,000,000 | $ 7,500,000 | $ 5,000,000 |  |  |  |

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