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Marketable Securities (Narrative) (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Dec. 31, 2022
Schedule of Investments [Line Items]          
Cash equivalents at fair value $ 470,591,000   $ 470,591,000   $ 450,240,000
Net unrealized loss 9,057,000   9,057,000   9,294,000
Net unrealized gains (losses) income tax expense (benefit) (2,705,000)   (2,705,000)   (2,777,000)
Available-for-sale securities, gross realized gains     0 $ 0  
Mutual funds and equities, gross realized gains     44,000 7,000  
Mutual funds and equities, gross realized loss     49,000 658,000  
Other long-term investments 54,318,000   54,318,000   104,673,000
Securities owned 31,040,000   31,040,000   32,148,000
Available-for-sale debt securities          
Schedule of Investments [Line Items]          
Available-for-sale securities, gross realized losses     122,000 414,000  
Investment Funds Sponsored by LSV          
Schedule of Investments [Line Items]          
Other long-term investments 6,772,000   6,772,000   $ 6,366,000
Unrealized gains (losses) on investments $ 235,000 $ (1,270,000) $ 406,000 $ (916,000)