Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Cash equivalents fair value	$ 462,624	$ 543,765
Net unrealized holding gain (loss)	1,551	(465)
Net unrealized holding gains (losses), income tax expense (benefit)	464	(140)
Equity securities, gross realized gains	1,100	297
Equity securities, gross realized losses	250	48
Investment funds sponsored by LSV	98,433	67,413
Securities owned	34,064	33,486
Available for sale debt securities
Schedule of Investments [Line Items]
Available-for-sale securities, gross realized losses	902	493
Available-for-sale securities, gross realized gains	0	0
Investment funds sponsored by LSV
Schedule of Investments [Line Items]
Investment funds sponsored by LSV	6,166	5,988
Gains (losses) from the change in fair value of the funds	$ 178	$ 1,101	$ (1,147)

Accumulated Other Comprehensive Income (Loss), Debt Securities, Available For Sale Securities Adjustment Tax

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Amount, after tax, of accumulated unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of long-term investments classified as other.

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