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Marketable Securities (Available for Sale and Equity Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, cost	$ 136,848	$ 127,681
Available for sale debt securities, fair value	126,185	118,752
Available for sale and equity securities, cost	173,139	163,409
Available for sale and equity securities, gross unrealized gains	1,556	952
Available for sale and equity securities, gross unrealized (losses)	(10,668)	(8,948)
Available for sale and equity securities, fair value	164,027	155,413
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, cost	136,848	127,681
Available for sale debt securities, gross unrealized gains	0	0
Available for sale debt securities, gross unrealized (losses)	(10,663)	(8,929)
Available for sale debt securities, fair value	126,185	118,752
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	30,956	30,427
Equity securities, FV-NI, gross unrealized gains	1,271	818
Equity securities, FV-NI, gross unrealized (losses)	(5)	(19)
Equity securities, FV-NI, fair value	32,222	31,226
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	5,335	5,301
Equity securities, FV-NI, gross unrealized gains	285	134
Equity securities, FV-NI, gross unrealized (losses)	0	0
Equity securities, FV-NI, fair value	$ 5,620	$ 5,435

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