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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment income earned | $ 40,446 | $ 33,375 | $ 119,295 | $ 95,326
Alternative Investments [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment income earned | 2,700 | 1,600 | 9,500 | 0
Investments accounted for under the equity method [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment (loss) income | (10,300) | (55,400) | (61,800) | 26,100
Realized gains (losses) | 43,300 | 245,600 | (261,000) | 1,186,800
Net change in unrealized appreciation (depreciation) | 1,072,000 | 117,800 | 3,186,300 | (1,132,800)
Net gain | $ 1,105,000 | $ 308,000 | $ 2,863,500 | $ 80,100

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