Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Available For Sale and Equity Securities, Cost	$ 138,588	$ 102,549
Available For Sale and Equity Securities, Gross Unrealized Gains	1,491	2,870
Available For Sale and Equity Securities, Gross Unrealized (Losses)	0	0
Available For Sale and Equity Securities, Fair Value	140,079	105,419
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, Cost	126,964	91,262
Available for sale debt securities, Gross Unrealized Gains	874	2,015
Available for sale debt securities, Gross Unrealized (Losses)	0	0
Available for sale debt securities, Fair Value	127,838	93,277
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	6,692	6,866
Equity securities, FV-NI, Gross Unrealized Gains	380	382
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	7,072	7,248
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	4,932	4,421
Equity securities, FV-NI, Gross Unrealized Gains	237	473
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	$ 5,169	$ 4,894

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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No definition available.

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

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No definition available.

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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), classified as current.

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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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