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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, 2014 | Jun. 30, 2013 | Jun. 30, 2014 | Jun. 30, 2013 | Jun. 30, 2014 Alternative Investments [Member] | Jun. 30, 2013 Alternative Investments [Member] | Jun. 30, 2014 Alternative Investments [Member] | Jun. 30, 2013 Alternative Investments [Member] | Mar. 31, 2014 Investments accounted for under the equity method [Member] | Mar. 31, 2013 Investments accounted for under the equity method [Member] | Mar. 31, 2014 Investments accounted for under the equity method [Member] | Mar. 31, 2013 Investments accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  | $ 22,800,000 | $ 46,800,000 | $ 85,700,000 | $ 255,000,000
Realized gains |  |  |  |  |  |  |  |  | 74,200,000 | (22,100,000) | 197,600,000 | 599,700,000
Net change in unrealized appreciation (depreciation) |  |  |  |  |  |  |  |  | 207,600,000 | 378,800,000 | 842,400,000 | (18,900,000)
Net income |  |  |  |  |  |  |  |  | 304,600,000 | 403,500,000 | 1,125,700,000 | 835,800,000
Net investment income earned | $ 36,774,000 | $ 34,003,000 | $ 72,308,000 | $ 66,873,000 | $ 3,600,000 | $ 3,900,000 | $ 8,800,000 | $ 7,500,000 |  |  |  |

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