Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Schedule of Investments [Line Items]
Net unrealized gains (losses) of available-for-sale debt securities	$ 1,261,000		$ 1,551,000
Unrealized gains (losses) during the period, income tax expense (benefit)	377,000		464,000
Available-for-sale securities, gross realized gains	0	$ 0
Mutual funds and equities, gross realized gains	157,000	189,000
Mutual funds and equities, gross realized loss	31,000	235,000
Other long-term investments	51,743,000		98,433,000
Securities owned	33,040,000		34,064,000
Cash Equivalents
Schedule of Investments [Line Items]
Cash equivalents at fair value	435,709,000		462,624,000
Available-for-sale debt securities
Schedule of Investments [Line Items]
Available-for-sale securities, gross realized losses	327,000	128,000
Investment Funds Sponsored by LSV
Schedule of Investments [Line Items]
Other long-term investments	6,623,000		$ 6,166,000
Unrealized gain (loss) on investments	$ 457,000	$ (2,235,000)

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

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Amount, after tax, of accumulated unrealized gain (loss) 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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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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