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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 |
--- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019
Schedule of Equity Method Investments [Line Items] |  |  |
Net investment income earned | $ 55,967 | $ 50,618 |
Carrying Value | 244,934 |  | $ 216,807
Alternative Investments [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Net investment income earned | 6,300 | 600 |
Carrying Value | 201,800 |  |
Investments accounted for under the equity method [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Net investment (loss) income | 12,700 | (150,000) |
Realized (losses) gains | 164,800 | 143,300 |
Net change in unrealized appreciation (depreciation) | 1,204,000 | 554,300 |
Net income | $ 1,381,500 | $ 547,600 |

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