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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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 55,826	$ 52,443	$ 164,949	$ 141,227
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned	5,200	7,100	13,000	10,600
Investments accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items]
Net investment (loss) income	(14,000)	11,900	10,100	(29,900)
Realized gains	121,100	124,800	370,400	1,348,300
Net change in unrealized appreciation (depreciation)	1,739,400	1,434,300	4,517,400	695,800
Net income	$ 1,846,500	$ 1,571,000	$ 4,897,900	$ 2,014,200

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