Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Schedule of Investments [Line Items]
Cash and cash equivalents at fair value	$ 425,697		$ 543,765
Net unrealized gains (losses) of available-for-sale debt securities	1,696		465
Unrealized gains (losses) during the period, income tax expense (benefit)	507		140
Mutual funds and equities, gross realized gains	189	$ 43
Mutual funds and equities, realized loss	235	3
Other Long-term Investments	45,285		67,413
Securities owned	31,420		33,486
Available-for-sale debt securities
Schedule of Investments [Line Items]
Available-for-sale securities, gross realized losses	128	109
Investment Funds Sponsored by LSV
Schedule of Investments [Line Items]
Other Long-term Investments	3,753		$ 5,988
Gains (losses) from investment funds sponsored by LSV	$ (2,235)	$ (452)

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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