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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
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 771,673		$ 771,673		$ 831,407
Restricted cash	351		351		351
Capitalized software development costs			13,809	$ 12,271
Net book value of capitalized software	$ 230,497		$ 230,497		243,446
Anti-dilutive employee stock options (in shares)	12,064	11,530	12,141	11,623
Anti-dilutive employee stock options (in USD per share)	$ 60.27	$ 57.77	$ 60.28	$ 57.77
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Net book value of capitalized software	$ 224,736		$ 224,736
Capitalized software in progress	28,744		28,744		29,253
Amortization expense related to the SEI Wealth Platform			$ 23,859	$ 23,876
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform			10 years 3 months 18 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	320,825		$ 320,825		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	$ 9,039	$ 11,256	$ 20,224	$ 21,489

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