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Marketable Securities (Available for Sale and Equity Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, cost	$ 149,579	$ 127,681
Available for sale debt securities, fair value	138,043	118,752
Available for sale and equity securities, cost	186,610	163,409
Available for sale and equity securities, gross unrealized gains	1,878	952
Available for sale and equity securities, gross unrealized (losses)	(11,539)	(8,948)
Available for sale and equity securities, fair value	176,949	155,413
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, cost	149,579	127,681
Available for sale debt securities, gross unrealized gains	0	0
Available for sale debt securities, gross unrealized (losses)	(11,536)	(8,929)
Available for sale debt securities, fair value	138,043	118,752
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	31,665	30,427
Equity securities, FV-NI, gross unrealized gains	1,533	818
Equity securities, FV-NI, gross unrealized (losses)	(3)	(19)
Equity securities, FV-NI, fair value	33,195	31,226
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	5,366	5,301
Equity securities, FV-NI, gross unrealized gains	345	134
Equity securities, FV-NI, gross unrealized (losses)	0	0
Equity securities, FV-NI, fair value	$ 5,711	$ 5,435

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