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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, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Sep. 30, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned | $ 57,594 | $ 55,826 | $ 58,505 | $ 50,618 | $ 54,109 | $ 52,443 | $ 45,553 | $ 43,231 | $ 222,543 |  | $ 195,336 |  | $ 161,882 |
Investments Accounted For Under The Equity Method [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment (loss) income |  |  |  |  |  |  |  |  |  | $ (8,000) |  | $ 134,000 |  | $ (143,000)
Realized gains |  |  |  |  |  |  |  |  |  | 695,000 |  | 1,981,000 |  | 325,000
Net change in unrealized appreciation |  |  |  |  |  |  |  |  |  | 5,543,000 |  | 1,303,000 |  | 2,894,000
Net Income |  |  |  |  |  |  |  |  |  | 6,230,000 |  | 3,418,000 |  | 3,076,000
Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  |  | $ 17,900 |  | $ 17,600 |  | $ 12,700

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