Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 746,870		$ 841,446
Restricted cash	3,101		3,101
Capitalized software development costs	6,458	$ 9,937
Net book value of capitalized software	$ 290,427		296,068
Anti-dilutive employee stock options (in shares)	7,408	6,323
Anti-dilutive employee stock options (in USD per share)	$ 58.76	$ 54.81
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs	$ 6,229	$ 9,739
Net book value of capitalized software	274,885
Capitalized software in progress	58,149		55,332
Amortization expense of capitalized software	$ 10,797	10,399
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Weighted Average Estimated Useful Life	8 years 4 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	$ 278,514		414,581
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
SEI-Sponsored Open-Ended Money Market Mutual Funds
Summary of Significant Accounting Policies [Line Items]
Fees waived	$ 6,641	$ 7,905