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Marketable Securities - Available for Sale and Equity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, cost	$ 142,718	$ 127,681
Available for sale debt securities, fair value	135,471	118,752
Available for sale and equity securities, cost	180,339	163,409
Available for sale and equity securities, gross unrealized gains	2,630	952
Available for sale and equity securities, gross unrealized (losses)	(7,247)	(8,948)
Available for sale and equity securities, fair value	175,722	155,413
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, cost	142,718	127,681
Available for sale debt securities, gross unrealized gains	0	0
Available for sale debt securities, gross unrealized (losses)	(7,247)	(8,929)
Available for sale debt securities, fair value	135,471	118,752
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	32,203	30,427
Equity securities, FV-NI, gross unrealized gains	2,144	818
Equity securities, FV-NI, gross unrealized (losses)	0	(19)
Equity securities, FV-NI, fair value	34,347	31,226
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, cost	5,418	5,301
Equity securities, FV-NI, gross unrealized gains	486	134
Equity securities, FV-NI, gross unrealized (losses)	0	0
Equity securities, FV-NI, fair value	$ 5,904	$ 5,435

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