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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 777,051		$ 777,051		$ 853,008
Restricted cash	300		300		351
Capitalized software development costs			18,036	$ 13,809
Net book value of capitalized software	$ 242,525		$ 242,525		237,302
Anti-dilutive employee stock options (in shares)	11,006	12,064	11,140	12,141
Anti-dilutive employee stock options (in USD per share)	$ 61.29	$ 60.27	$ 61.29	$ 60.28
Investment Managers
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs			$ 8,642
Capitalized software development costs in-progress			18,196		9,558
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Net book value of capitalized software	$ 222,853		222,853
Capitalized software in progress	$ 3,127		3,127		14,217
Amortization expense related to the SEI Wealth Platform			$ 12,516	$ 23,859
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform	9 years 4 months 24 days		9 years 4 months 24 days
SEI-Sponsored Open-Ended Money Market Mutual Funds
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 304,611		$ 304,611		305,434
SEI Investments (Europe) Limited
Summary of Significant Accounting Policies [Line Items]
Restricted cash	250		250		250
SEI Investments Distribution Co. (SIDCO)
Summary of Significant Accounting Policies [Line Items]
Restricted cash	50		50		$ 101
SEI-Sponsored Open-Ended Money Market Mutual Funds
Summary of Significant Accounting Policies [Line Items]
Fees waived	$ 5,320	$ 9,039	$ 10,881	$ 20,224

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