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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, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Sep. 30, 2015
Dec. 31, 2014
Sep. 30, 2014
Schedule of Equity Method Investments [Line Items]                            
Net investment income earned $ 35,428 $ 33,375 $ 31,182 $ 30,769 $ 30,108 $ 32,061 $ 32,230 $ 26,917 $ 130,754   $ 121,316   $ 138,708  
Investments Accounted For Under The Equity Method [Member]                            
Schedule of Equity Method Investments [Line Items]                            
Net Investment Income                   $ (44,000)   $ 129,000   $ 226,000
Realized gains                   1,374,000   1,187,000   581,000
Net change in unrealized (depreciation) appreciation                   (719,000)   (1,364,000)   1,098,000
Net Income                   $ 611,000   $ (48,000)   $ 1,905,000
Alternative Investments [Member]                            
Schedule of Equity Method Investments [Line Items]                            
Net investment income earned                 $ 3,100   $ (1,900)   $ 13,600