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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Jun. 30, 2015 | Sep. 30, 2014 | Jun. 30, 2014 | Sep. 30, 2015 | Jun. 30, 2015 | Sep. 30, 2014 | Jun. 30, 2014
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income earned | $ 32,061 |  | $ 34,292 |  | $ 91,208 |  | $ 106,600 |
Alternative Investments [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income earned | $ 1,300 |  | $ 3,900 |  | $ (800) |  | $ 12,700 |
Investments accounted for under the equity method [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net Investment Income |  | $ 44,100 |  | $ 81,300 |  | $ 139,600 |  | $ 167,000
Realized gains (losses) |  | 385,200 |  | (26,100) |  | 977,700 |  | 171,500
Net change in unrealized (depreciation) appreciation |  | (222,200) |  | 628,600 |  | (1,089,000) |  | 1,471,000
Net income |  | $ 207,100 |  | $ 683,800 |  | $ 28,300 |  | $ 1,809,500

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