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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, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Net investment income earned | $ 93,032 | $ 68,185 | $ 246,479 | $ 158,596 |
Carrying Value | 392,788 |  | 392,788 |  | $ 266,322
Alternative Investments [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Net investment income earned | 42,800 | 18,700 | 92,900 | 8,900 |
Investments accounted for under the equity method [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Net investment income | 60,400 | 83,400 | 550,300 | 92,900 |
Realized gains | 1,857,600 | 731,300 | 4,026,200 | 1,075,000 |
Net change in unrealized appreciation | 11,188,200 | 2,230,400 | 20,767,900 | 571,600 |
Insurance Subsidiaries' alternative investments income | $ 13,106,200 | $ 3,045,100 | $ 25,344,400 | $ 1,739,500 |

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