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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 |
--- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 695,701 | $ 679,661 | $ 695,701 | $ 679,661 | $ 667,446 | $ 578,273
Restricted cash | 3,500 | 5,500 | 3,500 | 5,500 |  |
Capitalized software development costs |  |  | 50,392 | 29,416 | $ 34,877 |
Net book value of capitalized software | 295,867 | $ 290,522 | $ 295,867 | $ 290,522 |  |
Anti-dilutive employee stock options |  |  | 10,632 | 10,730 | 10,166 |
Anti-dilutive employee stock options, average exercise price per share |  |  | $ 35.02 | $ 33.99 | $ 30.00 |
SEI Wealth Platform [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Capitalized software development costs |  |  | $ 39,785 | $ 24,515 | $ 34,877 |
Net book value of capitalized software | $ 280,359 |  | 280,359 |  |  |
Estimated useful life of the SEI Wealth Platform | 15 years |  |  |  |  |
Amortization expense related to the SEI Wealth Platform |  |  | 45,047 | 42,401 | $ 38,357 |
Estimated amortization expense related to the SEI Wealth Platform in 2017 | $ 48,166 |  | 48,166 |  |  |
Estimated amortization expense related to the SEI Wealth Platform in 2018 | 48,166 |  | 48,166 |  |  |
Estimated amortization expense related to the SEI Wealth Platform in 2019 | 48,166 |  | 48,166 |  |  |
Estimated amortization expense related to the SEI Wealth Platform in 2020 | 48,166 |  | 48,166 |  |  |
Estimated amortization expense related to the SEI Wealth Platform in 2021 | 48,166 |  | 48,166 |  |  |
Income statement classification for capitalized software write off related to the SEI Wealth Platform |  | Facilities, supplies and other costs |  |  |  |
Technological capabilities for Investment Managers segment [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Capitalized software development costs |  |  | 10,607 | 4,901 |  |
Net book value of capitalized software | 15,508 |  | 15,508 |  |  |
SEI Sponsored Open Ended Money Market Mutual Funds [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash and cash equivalents | 374,760 | $ 448,957 | 374,760 | 448,957 |  |
SEI Investments Europe Limited [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Restricted cash | 3,000 | 5,000 | 3,000 | 5,000 |  |
SEI Investments Distribution Co [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Restricted cash | $ 500 | 500 | $ 500 | $ 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 | 5 years 5 months 27 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 |  |  |  |

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