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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]        
Net investment income earned $ 37,419   $ 30,769  
Alternative Investments [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment income earned $ 1,600   $ (1,100)  
Investments accounted for under the equity method [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment (loss) income   $ 25,600   $ 46,600
Realized gains   (235,100)   752,500
Net change in unrealized depreciation   561,500   (883,200)
Net gain   $ 352,000   $ (84,100)