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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Available for sale and equity securities, cost	$ 130,843	$ 128,898
Available for sale and equity securities, gross unrealized gains	0	723
Available for sale and equity securities, gross unrealized (losses)	(9,161)	(80)
Available for sale and equity securities, fair value	121,682	129,541
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, cost	119,702	117,215
Available for sale debt securities, gross unrealized gains	0	0
Available for sale debt securities, gross unrealized (losses)	(8,832)	(80)
Available for sale debt securities, fair value	110,870	117,135
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	6,271	6,748
Equity securities, FV-NI, gross unrealized gains	0	463
Equity securities, FV-NI, gross unrealized (losses)	(249)	0
Equity securities, FV-NI, fair value	6,022	7,211
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	4,870	4,935
Equity securities, FV-NI, gross unrealized gains	0	260
Equity securities, FV-NI, gross unrealized (losses)	(80)	0
Equity securities, FV-NI, fair value	$ 4,790	$ 5,195

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