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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
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 791,437		$ 791,437		$ 831,407
Restricted cash	351		351		351
Capitalized software development costs			24,827	$ 19,486
Net book value of capitalized software	$ 234,228		$ 234,228		243,446
Anti-dilutive employee stock options (in shares)	11,862	11,347	12,048	11,531
Anti-dilutive employee stock options (in USD per share)	$ 60.26	$ 57.80	$ 60.27	$ 57.78
Investment Managers
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs			$ 5,284
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Net book value of capitalized software	$ 225,725		225,725
Capitalized software in progress	8,660		8,660		29,253
Amortization expense related to the SEI Wealth Platform			$ 29,696	$ 35,834
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform			10 years
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	305,877		$ 305,877		290,256
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	101		101		$ 101
SEI-Sponsored Open-Ended Money Market Mutual Funds
Summary of Significant Accounting Policies [Line Items]
Fees waived	$ 7,896	$ 11,629	$ 28,120	$ 33,118

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