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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 6 Months Ended
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2016
Mar. 31, 2016
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2016
Mar. 31, 2016
Schedule of Equity Method Investments [Line Items]                
Net investment income earned $ 41,430   $ 31,182   $ 78,849   $ 61,951  
Alternative Investments [Member]                
Schedule of Equity Method Investments [Line Items]                
Net investment income earned $ 5,200   $ (600)   $ 6,800   $ (1,700)  
Investments accounted for under the equity method [Member]                
Schedule of Equity Method Investments [Line Items]                
Net investment (loss) income   $ (9,900)   $ (4,600)   $ (62,400)   $ 37,000
Realized gains   (70,200)   193,200   (304,300)   981,100
Net change in unrealized depreciation   1,418,500   (253,900)   1,890,000   (1,236,500)
Net gain   $ 1,338,400   $ (65,300)   $ 1,523,300   $ (218,400)