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Investments (Aggregated Income Statement Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned | $ 68,511 | $ 68,185 | $ 34,444 | $ 55,967 | $ 57,594 | $ 55,826 | $ 58,505 | $ 50,618 | $ 227,107 |  | $ 222,543 |  | $ 195,336 |  |
Investments Accounted For Under The Equity Method [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment (loss) income |  |  |  |  |  |  |  |  |  | $ (26,000) |  | $ (8,000) |  | $ 134,000 |
Realized gains |  |  |  |  |  |  |  |  |  | 1,452,000 |  | 695,000 |  | 1,981,000 |
Net change in unrealized appreciation |  |  |  |  |  |  |  |  |  | 4,898,000 |  | 5,543,000 |  | 1,303,000 |
Net Income before tax |  |  |  |  |  |  |  |  |  | 6,324,000 |  | 6,230,000 |  | $ 3,418,000 |
Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  |  | $ 26,500 |  | $ 17,900 |  |  | $ 17,600

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