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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Summary of Significant Accounting Policies [Line Items]
Expense related to soft dollar arrangement	$ 3,265	$ 3,297	$ 10,497	$ 10,799
Cash and cash equivalents invested in SEI-sponsored money market funds	741,965		741,965		$ 744,247
Restricted cash	3,511		3,511		3,505
Capitalized software development costs			33,371	$ 48,573
Net book value of capitalized software	$ 310,146		$ 310,146		310,405
Anti-dilutive employee stock options (in shares)	6,183	11,324	6,153	11,286
Anti-dilutive employee stock options (in USD per share)	$ 53.38	$ 37.81	$ 53.15	$ 37.73
SEI Wealth Platform
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs			$ 32,526	$ 40,604
Net book value of capitalized software	$ 288,010		288,010
Capitalized software in progress	38,552		38,552
Amortization expense of capitalized software			$ 29,723	37,324
SEI Wealth Platform | Weighted Average
Summary of Significant Accounting Policies [Line Items]
Weighted Average Estimated Useful Life			8 years 4 months 24 days
Application for Investment Managers segment [Member]
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs			$ 845	$ 7,969
Net book value of capitalized software	22,136		$ 22,136
Weighted Average Estimated Useful Life			5 years
Amortization expense of capitalized software			$ 3,907
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	511		511		505
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	$ 286,824		$ 286,824		$ 401,292

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