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Investments (Aggregated Income Statement Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2021 | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Net investment income earned | $ 326,589 |  | $ 227,107 |  | $ 222,543 |
Investments Accounted For Under The Equity Method [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Net investment (loss) income |  | $ 653,000 |  | $ (26,000) |  | $ (8,000)
Realized gains |  | 6,121,000 |  | 1,452,000 |  | 695,000
Net change in unrealized appreciation |  | 26,877,000 |  | 4,898,000 |  | 5,543,000
Net Income before tax |  | 33,651,000 |  | 6,324,000 |  | 6,230,000
Alternative Investments [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Net investment income earned |  | $ 117,700 |  | $ 26,500 |  | $ 17,900

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