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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, 2019
Mar. 31, 2018
Schedule of Equity Method Investments [Line Items]    
Net investment income earned $ 50,618 $ 43,231
Alternative Investments [Member]    
Schedule of Equity Method Investments [Line Items]    
Net investment income earned 600 1,600
Investments accounted for under the equity method [Member]    
Schedule of Equity Method Investments [Line Items]    
Net investment income (loss) (150,100) (56,200)
Realized gains (losses) 143,300 1,003,000
Net change in unrealized appreciation 554,300 96,200
Net gain $ 547,500 $ 1,043,000