Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 834,355		$ 853,008
Restricted cash	352		351
Capitalized software development costs	9,041	$ 6,589
Net book value of capitalized software	$ 240,038		237,302
Anti-dilutive employee stock options (in shares)	11,273	12,218
Anti-dilutive employee stock options (in USD per share)	$ 61.28	$ 60.28
Investment Managers
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs	$ 4,307
Capitalized software development costs in-progress	13,864
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Net book value of capitalized software	224,551
Capitalized software in progress	10,586		14,217
Amortization expense related to the SEI Wealth Platform	$ 6,158	$ 12,104
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform	9 years 7 months 6 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	$ 341,216		305,434
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	102		$ 101
SEI-Sponsored Open-Ended Money Market Mutual Funds
Summary of Significant Accounting Policies [Line Items]
Fees waived	$ 5,561	$ 11,185