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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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]        
Net investment income earned $ 40,446 $ 33,375 $ 119,295 $ 95,326
Alternative Investments [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment income earned 2,700 1,600 9,500 0
Investments accounted for under the equity method [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment (loss) income (10,300) (55,400) (61,800) 26,100
Realized gains (losses) 43,300 245,600 (261,000) 1,186,800
Net change in unrealized appreciation (depreciation) 1,072,000 117,800 3,186,300 (1,132,800)
Net gain $ 1,105,000 $ 308,000 $ 2,863,500 $ 80,100