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Investments and Other Marketable Securities - Available-for-Sale and Equity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, Cost Amount	$ 172,986	$ 154,211
Available for sale debt securities, Fair Value	167,821	143,126
Available for sale and equity securities, Cost Amount	223,327	192,794
Available for sale and equity securities, Gross Unrealized Gains	3,153	1,947
Available for sale and equity securities, Gross Unrealized (Losses)	(5,165)	(11,085)
Available for sale and equity securities, Fair Value	221,315	183,656
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, Cost Amount	172,986	154,211
Available-for-sale debt securities, Gross Unrealized Gains	0	0
Available-for-sale debt securities, Gross Unrealized (Losses)	(5,165)	(11,085)
Available for sale debt securities, Fair Value	167,821	143,126
SEI-sponsored investment products
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost Amount	44,348	33,029
Equity securities, FV-NI, Gross Unrealized Gains	2,470	1,615
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	46,818	34,644
Equities and other investment products
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost Amount	5,993	5,554
Equity securities, FV-NI, Gross Unrealized Gains	683	332
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	$ 6,676	$ 5,886

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