Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Schedule of Investments [Line Items]
Cash and cash equivalents at fair value	$ 445,941,000		$ 445,941,000		$ 543,765,000
Net unrealized gains (losses) of available-for-sale debt securities	1,926,000		1,926,000		465,000
Unrealized gains (losses) during the period, income tax expense (benefit)	575,000		575,000		140,000
Available-for-sale securities, gross realized gains			0	$ 0
Mutual funds and equities, gross realized gains			196,000	56,000
Mutual funds and equities, realized loss			240,000	238,000
Other long-term investments	41,094,000		41,094,000		67,413,000
Securities owned	35,030,000		35,030,000		33,486,000
Available-for-sale debt securities
Schedule of Investments [Line Items]
Available-for-sale securities, gross realized losses			280,000	227,000
Investment Funds Sponsored by LSV
Schedule of Investments [Line Items]
Other long-term investments	4,749,000		4,749,000		$ 5,988,000
Unrealized gain (loss) on investments	$ 996,000	$ 95,000	$ (1,239,000)	$ 547,000