Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Investments [Line Items]
Cash and cash equivalents at fair value	$ 412,663,000		$ 412,663,000		$ 543,765,000
Unrealized gain (loss) on investments			0	$ 0
Net unrealized gains (losses) of available-for-sale debt securities	1,622,000		1,622,000		465,000
Unrealized gains (losses) during the period, income tax expense (benefit)	484,000		484,000		140,000
Available-for-sale securities, gross realized gains			0	0
Mutual funds and equities, gross realized gains			254,000
Mutual funds and equities, gross realized loss			250,000
Other long-term investments	33,117,000		33,117,000		67,413,000
Securities owned	35,820,000		35,820,000		33,486,000
Net gains or losses related to the securities	0	$ 0	0	0
Available-for-sale debt securities
Schedule of Investments [Line Items]
Available-for-sale securities, gross realized losses			582,000	347,000
Investment Funds Sponsored by LSV
Schedule of Investments [Line Items]
Unrealized gain (loss) on investments	458,000	$ 99,000	(781,000)	$ 646,000
Other long-term investments	$ 5,207,000		$ 5,207,000		$ 5,988,000