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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 |

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