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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 | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended
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Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012 | Jun. 30, 2013 Alternative Investments [Member] | Jun. 30, 2012 Alternative Investments [Member] | Jun. 30, 2013 Alternative Investments [Member] | Jun. 30, 2012 Alternative Investments [Member] | Jun. 30, 2013 Investments accounted for under the equity method [Member] | Jun. 30, 2012 Investments accounted for under the equity method [Member] | Jun. 30, 2013 Investments accounted for under the equity method [Member] | Jun. 30, 2012 Investments accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  | $ 97,800,000 | $ 80,000,000 | $ 352,800,000 | $ 170,100,000
Realized (losses) gains |  |  |  |  |  |  |  |  | 162,800,000 | 19,600,000 | 762,500,000 | 1,004,900,000
Net change in unrealized depreciation |  |  |  |  |  |  |  |  | 104,800,000 | (117,100,000) | 85,900,000 | (551,100,000)
Net income |  |  |  |  |  |  |  |  | 365,400,000 | (17,500,000) | 1,201,200,000 | 623,900,000
Net investment income earned | $ 32,457,000 | $ 30,650,000 | $ 99,330,000 | $ 97,284,000 | $ 2,600,000 | $ 500,000 | $ 10,100,000 | $ 5,500,000 |  |  |  |

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