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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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Summary of Significant Accounting Policies [Line Items]
Operating Lease Right-of-Use Assets	$ 42,940		$ 42,940
Lease liabilities	47,686		47,686
Cash and cash equivalents invested in SEI-sponsored money market funds	682,266		682,266			$ 754,525
Restricted cash	3,520		3,520			3,514
Capitalized software development costs			19,188	$ 24,613
Net book value of capitalized software	$ 305,184		$ 305,184			309,500
Anti-dilutive employee stock options (in shares)	6,244	6,221	6,284	6,138
Anti-dilutive employee stock options (in USD per share)	$ 54.79	$ 53.33	$ 54.80	$ 53.03
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs			$ 18,693	$ 23,768
Net book value of capitalized software	$ 286,459		286,459
Capitalized software in progress	49,374		49,374			42,238
Amortization expense of capitalized software			$ 20,900	19,599
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Weighted Average Estimated Useful Life			8 years 6 months
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	272,986		$ 272,986			315,840
SEI Investments (Europe) Limited
Summary of Significant Accounting Policies [Line Items]
Restricted cash	3,000		3,000			3,000
SEI Investments Distribution Co. (SIDCO)
Summary of Significant Accounting Policies [Line Items]
Restricted cash	520		520			$ 514
SEI-Sponsored Open-Ended Money Market Mutual Funds
Summary of Significant Accounting Policies [Line Items]
Fees waived according to expense limitation agreements	$ 6,796	$ 6,372	$ 14,701	$ 13,026
Accounting Standards Update 2016-02
Summary of Significant Accounting Policies [Line Items]
Operating Lease Right-of-Use Assets					$ 44,169
Lease liabilities					$ 44,169

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