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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, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Schedule of Equity Method Investments [Line Items] |  |  |
Net investment income earned | $ 69,716 | $ 55,967 |
Carrying Value | 290,153 |  | $ 266,322
Alternative Investments [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Net investment income earned | 20,200 | 6,300 |
Investments accounted for under the equity method [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Net investment income | 481,600 | 12,700 |
Realized gains | 776,000 | 164,800 |
Net change in unrealized appreciation | 4,630,800 | 1,204,000 |
Insurance Subsidiaries' alternative investments income | $ 5,888,400 | $ 1,381,500 |

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