Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Investments Available for sale, Cost Amount	$ 113,763	$ 115,394
Investments Available for sale, Gross Unrealized Gains	3,078	1,562
Investments Available for sale, Gross Unrealized (Losses)	(277)	(39)
Investments Available for sale, Fair Value	116,564	116,917
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Debt securities, Cost Amount	103,365	104,193
Debt securities, Gross Unrealized Gains	2,106	605
Debt securities, Gross Unrealized (Losses)	0	0
Debt securities, Fair Value	105,471	104,798
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost Amount	6,793	7,564
Equity securities, FV-NI, Gross Unrealized Gains	63	125
Equity securities, FV-NI, Gross Unrealized (Losses)	(277)	(39)
Equity securities, FV-NI	6,579	7,650
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost Amount	3,605	3,637
Equity securities, FV-NI, Gross Unrealized Gains	909	832
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI	$ 4,514	$ 4,469

Debt Securities, Available-For-Sale And Equity Securities, FV-NI

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Debt Securities, Available-For-Sale And Equity Securities, FV-NI, Accumulated Gross Unrealized Gain, Before Tax

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Debt Securities, Available-For-Sale And Equity Securities, FV-NI, Accumulated Gross Unrealized Loss, Before Tax

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Debt Securities, Available-For-Sale And Equity Securities, FV-NI, Cost

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Equity Securities, FV-NI, Accumulated Gross Unrealized Gain, Before Tax

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Equity Securities, FV-NI, Accumulated Gross Unrealized Loss, Before Tax

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) 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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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) 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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Amortized cost of 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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Amount of 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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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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