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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

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