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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
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