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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 907,831		$ 831,407
Restricted cash	351		351
Capitalized software development costs	6,589	$ 6,360
Net book value of capitalized software	$ 236,481		243,446
Anti-dilutive employee stock options (in shares)	12,218	11,715
Anti-dilutive employee stock options (in USD per share)	$ 60.28	$ 57.99
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Net book value of capitalized software	$ 230,363
Capitalized software in progress	31,108		29,253
Amortization expense related to the SEI Wealth Platform	$ 12,104	$ 11,923
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform	10 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	$ 299,843		290,256
SEI Investments (Europe) Limited
Summary of Significant Accounting Policies [Line Items]
Restricted cash	250		250
SEI Investments Distribution Co. (SIDCO)
Summary of Significant Accounting Policies [Line Items]
Restricted cash	101		$ 101
SEI-Sponsored Open-Ended Money Market Mutual Funds
Summary of Significant Accounting Policies [Line Items]
Fees waived	$ 11,185	$ 10,233

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