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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 854,443		$ 854,443		$ 853,008
Restricted cash	301		301		351
Net book value of capitalized software	$ 244,416		$ 244,416		237,302
Anti-dilutive employee stock options (in shares)	10,869	11,862	11,049	12,048
Anti-dilutive employee stock options (in USD per share)	$ 61.31	$ 60.26	$ 61.29	$ 60.27
Investment Managers
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs			$ 12,878
Capitalized software development costs in-progress			22,522		9,558
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs			13,706	$ 19,543
Net book value of capitalized software	$ 220,654		220,654
Capitalized software in progress	$ 1,439		1,439		14,217
Amortization expense related to the SEI Wealth Platform			$ 19,027	29,696
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform	9 years 2 months 12 days		9 years 2 months 12 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	$ 424,891		$ 424,891		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	$ 5,519	$ 7,896	$ 16,400	$ 28,120

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