Investments (Narrative) (Details)		3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 25, 2017 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) investment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Impairment loss on equity securities without readily determinable fair value		$ 0			$ 19,600,000		$ 4,200,000
Non-cash exchange of available-for-sale debt securities			$ 18,300,000
Recognized loss on exchange of available-for-sale debt securities		0			4,400,000
Reported Amount		29,896,000			29,896,000		$ 27,135,000
Recognized gain on exchange of available-for-sale debt securities		0			3,200,000
Variable interest entity, amount committed to invest	$ 200,000,000
Variable interest entity, ownership percentage	76.60%
Annual management fee percentage	2.00%
Management fee annual reduction percentage	10.00%
Entitled carried interest percentage	20.00%
Variable interest entity, amount of capital call notices received					31,000,000.0	$ 22,300,000
Variable interest entity, amount paid					30,000,000.0	22,300,000
Distribution from equity method investment					0	10,288,000
Income (loss) from equity method investment		(709,000)		$ (3,481,000)	(10,799,000)	126,000
Management fees recognized		$ 800,000		$ 800,000	2,300,000	$ 2,300,000
COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Income (loss) from equity method investment					$ (3,800,000)
Number of investments impaired | investment					2
Equity method investment, impairment					$ 7,000,000.0
Preferred Stock
Unusual or Infrequent Item, or Both [Line Items]
Reported Amount			$ 800,000