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Marketable Securities (Narrative) (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Schedule of Investments [Line Items]          
Cash equivalents at fair value $ 588,227,000   $ 588,227,000   $ 450,240,000
Net unrealized loss 11,111,000   11,111,000   9,294,000
Net unrealized gains (losses) income tax expense (benefit) (3,319,000)   (3,319,000)   (2,777,000)
Available-for-sale securities, gross realized gains     0 $ 0  
Mutual funds and equities, gross realized gains     63,000 226,000  
Mutual funds and equities, gross realized loss     101,000 714,000  
Other long-term investments 44,645,000   44,645,000   104,673,000
Securities owned 32,748,000   32,748,000   32,148,000
Available-for-sale debt securities          
Schedule of Investments [Line Items]          
Available-for-sale securities, gross realized losses     167,000 521,000  
Investment Funds Sponsored by LSV          
Schedule of Investments [Line Items]          
Other long-term investments 6,838,000   6,838,000   $ 6,366,000
Unrealized gains (losses) on investments $ 66,000 $ (380,000) $ 472,000 $ (1,296,000)