Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Investments [Line Items]
Net unrealized losses	$ 2,080		$ 779
Unrealized losses during the period, income tax benefit	621		233
Available-for-sale securities, gross realized gains	1,031
Available-for-sale securities, gross realized losses	1,520
Available-for-sale securities, gross realized gains		$ 428
Available-for-sale securities, gross realized losses		706
Investments in Affiliated Funds, at fair value	5,736		6,034
Securities owned	28,945		21,526
Investment Funds Sponsored by LSV
Schedule of Investments [Line Items]
Investments in Affiliated Funds, at fair value	5,736		$ 6,034
Gains (losses) from investment funds sponsored by LSV	$ (298)	$ 880

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

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Debt Securities, Available-For-Sale And Equity Securities, FN-NI, Realized Gain

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Debt Securities, Available-For-Sale And Equity Securities, FN-NI, Realized Loss

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Accumulated unrealized gain (loss), after tax, 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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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Amount 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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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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