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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, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]        
Net investment income earned $ 52,443 $ 40,446 $ 141,227 $ 119,295
Alternative Investments [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment income earned 7,100 2,700 10,600 9,500
Investments accounted for under the equity method [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment income (loss) 11,900 600 (29,900) (61,800)
Realized gains (losses) 124,800 43,300 1,348,300 (261,000)
Net change in unrealized appreciation 1,434,300 1,296,300 695,800 3,186,300
Net gain $ 1,571,000 $ 1,340,200 $ 2,014,200 $ 2,863,500