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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, 2018
 [1]
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
 [1]
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Sep. 30, 2015
Schedule of Equity Method Investments [Line Items]                            
Net investment income earned $ 54,109 $ 52,443 $ 45,553 $ 43,231 $ 42,587 $ 40,446 $ 41,430 $ 37,419 $ 195,336 $ 161,882   $ 130,754    
Investments Accounted For Under The Equity Method [Member]                            
Schedule of Equity Method Investments [Line Items]                            
Net investment income (loss)                     $ 134,000   $ (143,000) $ (44,000)
Realized gains                     1,981,000   325,000 1,374,000
Net change in unrealized appreciation (depreciation)                     1,303,000   2,894,000 (719,000)
Net Income                     $ 3,418,000   $ 3,076,000 $ 611,000
Alternative Investments [Member]                            
Schedule of Equity Method Investments [Line Items]                            
Net investment income earned                 $ 17,600 $ 12,700   $ 3,100    
[1] Results for the fourth quarter of 2017 include the impact of the $20.2 million write off of deferred tax assets required with the implementation of Tax Reform. See Note 13. "Federal Income Taxes" above for additional information.