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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Available For Sale and Equity Securities, Cost	$ 112,346	$ 102,549
Available For Sale and Equity Securities, Gross Unrealized Gains	2,627	2,870
Available For Sale and Equity Securities, Gross Unrealized (Losses)	0	0
Available For Sale and Equity Securities, Fair Value	114,973	105,419
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, Cost	100,980	91,262
Available for sale debt securities, Gross Unrealized Gains	1,638	2,015
Available for sale debt securities, Gross Unrealized (Losses)	0	0
Available for sale debt securities, Fair Value	102,618	93,277
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	6,872	6,866
Equity securities, FV-NI, Gross Unrealized Gains	446	382
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	7,318	7,248
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	4,494	4,421
Equity securities, FV-NI, Gross Unrealized Gains	543	473
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	$ 5,037	$ 4,894

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