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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, 2021	Mar. 31, 2020	Dec. 31, 2020
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 782,582		$ 784,626
Restricted cash	3,101		3,101
Capitalized software development costs	6,360	$ 6,458
Net book value of capitalized software	$ 263,964		270,977
Anti-dilutive employee stock options (in shares)	11,715	7,408
Anti-dilutive employee stock options (in USD per share)	$ 57.99	$ 58.76
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs	$ 6,261	$ 6,229
Net book value of capitalized software	252,047
Capitalized software in progress	17,455		13,409
Amortization expense of capitalized software	$ 11,923	10,797
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Weighted average remaining life	9 years 10 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	$ 323,941		347,082
SEI Investments (Europe) Limited
Summary of Significant Accounting Policies [Line Items]
Restricted cash	3,000		3,000
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	$ 10,233	$ 6,641

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