Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Cash equivalents fair value	$ 543,765	$ 419,154
Net unrealized holding gain (loss)	465	1,413
Net unrealized holding gains (losses), income tax expense (benefit)	140	422
Equity securities, gross realized gains	297	1,065
Equity securities, gross realized losses	48	1,136
Investment funds sponsored by LSV	67,413	52,342
Securities owned	33,486	30,892
Available for sale debt securities
Schedule of Investments [Line Items]
Available-for-sale securities, gross realized losses	493	550
Investment funds sponsored by LSV
Schedule of Investments [Line Items]
Investment funds sponsored by LSV	5,988	4,887
Change in fair value of investment funds sponsored by LSV	$ 1,101	$ (1,147)	$ 1,176

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 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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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 realized loss 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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The fair value as of the balance sheet date of firm holdings in debt obligations issued by the US government including short-term Treasury bills, medium-term Treasury notes, and long-term Treasury bonds, as well as debt issued by agencies. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Amount of long-term investments classified as other.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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