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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, 2020	Mar. 31, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 55,967	$ 50,618
Carrying Value	244,934		$ 216,807
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned	6,300	600
Carrying Value	201,800
Investments accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items]
Net investment (loss) income	12,700	(150,000)
Realized (losses) gains	164,800	143,300
Net change in unrealized appreciation (depreciation)	1,204,000	554,300
Net income	$ 1,381,500	$ 547,600

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