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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Available For Sale and Equity Securities, Cost	$ 102,549	$ 115,394
Available For Sale and Equity Securities, Gross Unrealized Gains	2,870	1,562
Available For Sale and Equity Securities, Gross Unrealized (Losses)	0	(39)
Available For Sale and Equity Securities, Fair Value	105,419	116,917
Available for sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, Cost	91,262	104,193
Available for sale debt securities, Gross Unrealized Gains	2,015	605
Available for sale debt securities, Gross Unrealized (Losses)	0	0
Available for sale debt securities, Fair Value	93,277	104,798
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	6,866	7,564
Equity securities, FV-NI, Gross Unrealized Gains	382	125
Equity securities, FV-NI, Gross Unrealized (Losses)	0	(39)
Equity securities, FV-NI, Fair Value	7,248	7,650
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	4,421	3,637
Equity securities, FV-NI, Gross Unrealized Gains	473	832
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	$ 4,894	$ 4,469

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