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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2019
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents | $ 841,446,000 |  |  |  | $ 754,525,000 |  |  |  | $ 841,446,000 | $ 754,525,000 |  |
Restricted cash | 3,101,000 |  |  |  | 3,514,000 |  |  |  | 3,101,000 | 3,514,000 |  |
Impairment charges related to marketable securities |  |  |  |  |  |  |  |  | 0 | 0 | $ 0 |
Capitalized software development costs |  |  |  |  |  |  |  |  | 34,074,000 | 44,221,000 | 61,043,000 |
Net book value of capitalized software | 296,068,000 |  |  |  | 309,500,000 |  |  |  | 296,068,000 | 309,500,000 |  |
Increase in net income from change in estimate | $ 128,737,000 | $ 132,168,000 | $ 126,540,000 | $ 113,981,000 | $ 116,034,000 | $ 128,319,000 | $ 121,677,000 | $ 139,838,000 | $ 501,426,000 | $ 505,868,000 | $ 404,389,000 |
Increase in diluted earnings per share from change in estimate | $ 0.84 | $ 0.86 | $ 0.82 | $ 0.73 | $ 0.73 | $ 0.80 | $ 0.75 | $ 0.86 | $ 3.24 | $ 3.14 | $ 2.49 |
Impairment charges related to capitalized software development costs |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 0 |
Goodwill and intangible assets impairment |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 0 |
Anti-dilutive employee stock options (in shares) |  |  |  |  |  |  |  |  | 6,574 | 6,224 | 5,196 |
Anti-dilutive employee stock options, average exercise price per share |  |  |  |  |  |  |  |  | $ 55.95 | $ 53.60 | $ 45.49 |
Operating lease, right-of-use asset | $ 42,789,000 |  |  |  |  |  |  |  | $ 42,789,000 |  |  |
Operating lease liability | 47,606,000 |  |  |  |  |  |  |  | 47,606,000 |  |  |
SEI Wealth Platform |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capitalized software development costs |  |  |  |  |  |  |  |  | 33,084,000 | $ 43,376,000 | $ 51,353,000 |
Net book value of capitalized software | 279,453,000 |  |  |  |  |  |  |  | 279,453,000 |  |  |
Capitalized software development costs in progress | 55,332,000 |  |  |  |  |  |  |  | 55,332,000 |  |  |
Amortization expense related to the SEI Wealth Platform |  |  |  |  |  |  |  |  | 42,297,000 | 39,917,000 | 46,505,000 |
Application for the Investment Managers segment |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capitalized software development costs |  |  |  |  |  |  |  |  |  | 845,000 | 9,690,000 |
Net book value of capitalized software | 15,625,000 |  |  |  |  |  |  |  | $ 15,625,000 |  |  |
Estimated useful life of the SEI Wealth Platform |  |  |  |  |  |  |  |  | 3 years |  |  |
Amortization expense related to the SEI Wealth Platform |  |  |  |  |  |  |  |  | $ 5,209,000 | 5,000 |  |
Remaining Useful Life of SEI Wealth Platform | SEI Wealth Platform |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Decrease in amortization expense from change in estimate |  |  |  |  |  |  |  |  |  |  | 4,347,000 |
Increase in net income from change in estimate |  |  |  |  |  |  |  |  |  |  | $ 3,156,000 |
Increase in diluted earnings per share from change in estimate |  |  |  |  |  |  |  |  |  |  | $ 0.02 |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating lease, right-of-use asset |  |  |  |  |  |  |  |  |  |  |  | $ 44,169,000
Operating lease liability |  |  |  |  |  |  |  |  |  |  |  | $ 44,169,000
Minimum | Building and Building Improvements |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 25 years |  |  |
Minimum | Computer Equipment |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 3 years |  |  |
Minimum | Furniture and fixtures |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 3 years |  |  |
Maximum | Building and Building Improvements |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 39 years |  |  |
Maximum | Computer Equipment |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 5 years |  |  |
Maximum | Furniture and fixtures |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of property and equipment |  |  |  |  |  |  |  |  | 5 years |  |  |
Weighted Average | SEI Wealth Platform |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of the SEI Wealth Platform |  |  |  |  |  |  |  |  | 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 | 414,581,000 |  |  |  | $ 315,840,000 |  |  |  | $ 414,581,000 | 315,840,000 |  |
SEI Investments Europe Limited |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash | 3,000,000 |  |  |  | 3,000,000 |  |  |  | 3,000,000 | 3,000,000 |  |
SEI Investments Distribution Co |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash | $ 101,000 |  |  |  | $ 514,000 |  |  |  | 101,000 | 514,000 |  |
SEI Sponsored Open Ended Money Market Mutual Funds |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Fees waived according to expense limitation agreements |  |  |  |  |  |  |  |  | $ 27,544,000 | $ 26,229,000 | $ 27,434,000 |

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