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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
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Sep. 30, 2016 | Jun. 30, 2016 | Sep. 30, 2015 | Jun. 30, 2015 | Sep. 30, 2016 | Jun. 30, 2016 | Sep. 30, 2015 | Jun. 30, 2015
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income earned | $ 33,375 |  | $ 32,061 |  | $ 95,326 |  | $ 91,208 |
Alternative Investments [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment income earned | $ 1,600 |  | $ 1,300 |  | $ 0 |  | $ (800) |
Investments accounted for under the equity method [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net investment (loss) income |  | $ (55,400) |  | $ 44,100 |  | $ 26,100 |  | $ 139,600
Realized gains |  | 245,600 |  | 385,200 |  | 1,186,800 |  | 977,700
Net change in unrealized depreciation |  | 117,800 |  | (222,200) |  | (1,132,800) |  | (1,089,000)
Net gain |  | $ 308,000 |  | $ 207,100 |  | $ 80,100 |  | $ 28,300

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