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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, 2023 | Sep. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022 | Dec. 31, 2021 | Sep. 30, 2021
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Net investment income earned | $ 388,650 |  | $ 288,155 |  | $ 326,589 |
Investments Accounted For Under The Equity Method [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Net investment income (loss) |  | $ 178,000 |  | $ 765,000 |  | $ 653,000
Realized gains |  | 5,845,000 |  | 12,590,000 |  | 6,121,000
Net change in unrealized appreciation (depreciation) |  | 5,810,000 |  | (5,215,000) |  | 26,877,000
Net Income |  | 11,833,000 |  | 8,140,000 |  | 33,651,000
Alternative Investments [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Net investment income earned | $ 26,777 | $ 26,800 | $ 23,003 | $ 23,000 | $ 117,701 | $ 117,700

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