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Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Schedule of Investments [Line Items]
Cash equivalents at fair value	$ 503,582,000		$ 450,240,000
Net unrealized loss	7,972,000		9,294,000
Net unrealized gains (losses) income tax expense (benefit)	(2,381,000)		(2,777,000)
Available-for-sale securities, gross realized gains	0	$ 0
Mutual funds and equities, gross realized gains	32,000	5,000
Mutual funds and equities, gross realized loss	10,000	579,000
Other long-term investments	55,041,000		104,673,000
Securities owned	31,779,000		32,148,000
Available-for-sale debt securities
Schedule of Investments [Line Items]
Available-for-sale securities, gross realized losses	46,000	246,000
Investment Funds Sponsored by LSV
Schedule of Investments [Line Items]
Other long-term investments	6,537,000		$ 6,366,000
Unrealized gains on investments	$ 171,000	$ 354,000

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