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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
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Mar. 31, 2018 | Mar. 31, 2017
Schedule of Equity Method Investments [Line Items] |  |
Net investment income earned | $ 43,231 | $ 37,419
Alternative Investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Net investment income earned | 1,600 | 1,600
Investments accounted for under the equity method [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Net investment (loss) income | (35,400) | 25,600
Realized gains (losses) | 594,000 | (235,100)
Net change in unrealized appreciation (depreciation) | 461,600 | 561,500
Net gain | $ 1,020,200 | $ 352,000

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