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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Available For Sale and Equity Securities, Cost	$ 138,994	$ 128,898
Available For Sale and Equity Securities, Gross Unrealized Gains	734	723
Available For Sale and Equity Securities, Gross Unrealized (Losses)	(5,011)	(80)
Available For Sale and Equity Securities, Fair Value	134,717	129,541
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, Cost	127,804	117,215
Available for sale debt securities, Gross Unrealized Gains	0	0
Available for sale debt securities, Gross Unrealized (Losses)	(4,990)	(80)
Available for sale debt securities, Fair Value	122,814	117,135
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	6,353	6,748
Equity securities, FV-NI, Gross Unrealized Gains	505	463
Equity securities, FV-NI, Gross Unrealized (Losses)	(21)	0
Equity securities, FV-NI, Fair Value	6,837	7,211
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	4,837	4,935
Equity securities, FV-NI, Gross Unrealized Gains	229	260
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	$ 5,066	$ 5,195

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