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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 68,185	$ 55,826	$ 158,596	$ 164,949
Carrying Value	258,648		258,648		$ 216,807
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned	18,700	5,200	8,900	13,000
Investments accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items]
Net investment (loss) income	83,400	(14,000)	92,900	10,100
Realized (losses) gains	731,300	121,100	1,075,000	370,400
Net change in unrealized appreciation (depreciation)	2,230,400	1,739,400	571,600	4,517,400
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 3,045,100	$ 1,846,500	$ 1,739,500	$ 4,897,900