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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands | 3 Months Ended | 12 Months Ended |
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Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents | $ 744,247 |  |  |  | $ 695,701 |  |  |  | $ 744,247 | $ 695,701 | $ 679,661 | $ 667,446
Restricted cash | 3,505 |  |  |  | 3,500 |  |  |  | 3,505 | 3,500 |  |
Capitalized software development costs |  |  |  |  |  |  |  |  | 61,043 | 50,392 | 29,416 |
Net book value of capitalized software | $ 310,405 |  |  |  | $ 295,867 |  |  |  | 310,405 | 295,867 |  |
Increase in income from operations from change in estimate |  |  |  |  |  |  |  |  | 396,944 | 375,694 | 358,213 |
Increase in net income from change in estimate |  |  |  |  |  |  |  |  | $ 404,389 | $ 333,817 | $ 331,655 |
Increase in diluted earnings per share from change in estimate | $ 0.75 | $ 0.63 | $ 0.57 | $ 0.55 | $ 0.55 | $ 0.53 | $ 0.49 | $ 0.47 | $ 2.49 | $ 2.03 | $ 1.96 |
Estimated amortization expense related to all capitalized software in 2018 | $ 43,717 |  |  |  |  |  |  |  | $ 43,717 |  |  |
Estimated amortization expense related to all capitalized software in 2019 | 43,717 |  |  |  |  |  |  |  | 43,717 |  |  |
Estimated amortization expense related to all capitalized software in 2020 | 43,717 |  |  |  |  |  |  |  | 43,717 |  |  |
Estimated amortization expense related to all capitalizes software in 2021 | 43,439 |  |  |  |  |  |  |  | 43,439 |  |  |
Estimated amortization expense related to all capitalized software in 2022 | 29,556 |  |  |  |  |  |  |  | $ 29,556 |  |  |
Anti-dilutive employee stock options |  |  |  |  |  |  |  |  | 5,196 | 10,632 | 10,730 |
Anti-dilutive employee stock options, average exercise price per share |  |  |  |  |  |  |  |  | $ 45.49 | $ 35.02 | $ 33.99 |
SEI Wealth Platform [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capitalized software development costs |  |  |  |  |  |  |  |  | $ 51,353 | $ 39,785 | $ 24,515 |
Net book value of capitalized software | 285,207 |  |  |  |  |  |  |  | 285,207 |  |  |
Capitalized software development costs in progress | 38,093 |  |  |  |  |  |  |  | $ 38,093 |  |  |
Estimated useful life of the SEI Wealth Platform |  |  |  |  |  |  |  |  | 15 years |  |  |
Amortization expense related to the SEI Wealth Platform |  |  |  |  |  |  |  |  | $ 46,505 | 45,047 | $ 42,401 |
Income statement classification for capitalized software write off related to the SEI Wealth Platform |  |  |  |  |  |  |  |  |  |  | Facilities, supplies and other costs |
Application for the Investment Managers segment [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capitalized software development costs |  |  |  |  |  |  |  |  | 9,690 | 10,607 | $ 4,901 |
Net book value of capitalized software | 25,198 |  |  |  |  |  |  |  | 25,198 |  |  |
SEI Investments Europe Limited [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash | 3,000 |  |  |  | $ 3,000 |  |  |  | 3,000 | 3,000 |  |
SEI Investments Distribution Co [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash | $ 505 |  |  |  | 500 |  |  |  | $ 505 | 500 |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest in unconsolidated affiliate | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |
Weighted Average [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of the SEI Wealth Platform | 8 years 2 months 12 days |  |  |  |  |  |  |  |  |  |  |
Building and Building Improvements [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 25 years |  |  |
Building and Building Improvements [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 39 years |  |  |
Computer Equipment [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 3 years |  |  |
Computer Equipment [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 5 years |  |  |
Furniture and Fixtures [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 3 years |  |  |
Furniture and Fixtures [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 5 years |  |  |
Equity Method Investee [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest in unconsolidated affiliate | 20.00% |  |  |  |  |  |  |  | 20.00% |  |  |
Equity Method Investee [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest in unconsolidated affiliate | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |
Capitalized software [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Loss on write off of capitalized software related to the SEI Wealth Platform |  |  |  |  |  |  |  |  |  |  | $ 5,533 |
Private Banks [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Segment classification for capitalized software write off related to the SEI Wealth Platform |  |  |  |  |  |  |  |  |  |  | Private Banks |
Investment Advisors [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Segment classification for capitalized software write off related to the SEI Wealth Platform |  |  |  |  |  |  |  |  |  |  | Investment Advisors |
SEI Sponsored Open Ended Money Market Mutual Funds [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents | $ 401,292 |  |  |  | $ 374,760 |  |  |  | $ 401,292 | $ 374,760 |  |
Remaining Useful Life of SEI Wealth Platform [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease in amortization expense from change in estimate |  |  |  |  |  |  |  |  | 4,347 |  |  |
Increase in income from operations from change in estimate |  |  |  |  |  |  |  |  | 4,347 |  |  |
Increase in net income from change in estimate |  |  |  |  |  |  |  |  | $ 3,156 |  |  |
Increase in diluted earnings per share from change in estimate |  |  |  |  |  |  |  |  | $ 0.02 |  |  |

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