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Shareholders' Equity (Narrative) (Details) |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 15, 2023 $ / shares | May 31, 2023 $ / shares | Dec. 31, 2023 USD ($) grantee $ / shares shares | Dec. 31, 2022 USD ($) $ / shares shares | Dec. 31, 2021 USD ($) $ / shares | Dec. 29, 2023 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Weighted average fair value of stock options granted (in USD per share) | $ / shares |  |  | $ 15.70 | $ 17.95 | $ 14.94 |
(Decrease) increase in stock-based compensation from change in management's estimate |  |  | $ 6,941,000 | $ 4,915,000 | $ (5,880,000) |
Unvested employee stock options outstanding (in shares) | shares |  |  | 10,661,000 |  |  |
Unrecognized compensation cost from employee stock options |  |  | $ 89,130,000 |  |  |
Weighted average period for recognition of unrecognized compensation cost |  |  | 1 year 9 months 18 days |  |  |
Exercisable (in shares) | shares |  |  | 6,759,000 | 9,043,000 |  |
Options outstanding weighted average remaining contractual life, years |  |  | 6 years 3 months 18 days |  |  |
Total intrinsic value of options exercised |  |  | $ 34,499,000 | $ 36,576,000 |  |
Aggregate intrinsic value of options exercisable |  |  | 64,577,000 |  |  |
Total intrinsic value of options outstanding |  |  | $ 103,189,000 |  |  |
Share price of SEI common stock (in USD per share) | $ / shares |  |  |  |  |  | $ 63.55
Dividends declared per common share (in USD per share) | $ / shares | $ 0.46 | $ 0.43 | $ 0.89 | $ 0.83 | $ 0.77 |
Dividends declared |  |  | $ 117,894,000 | $ 112,429,000 | $ 107,840,000 |
Restricted Stock Units (RSUs) |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Weighted average period for recognition of unrecognized compensation cost |  |  | 1 year 9 months 10 days |  |  |
Unrecognized compensation cost from employee stock options |  |  | $ 39,905,000 |  |  |
Share-Based Payment Arrangement, Option | Voluntary Separation Program |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of grantees affected | grantee |  |  | 52 |  |  |
Voluntary seperation program expense |  |  | $ 2,532,000 |  |  |
Voluntary seperation program reversal |  |  | $ 1,783,000 | 3,562,000 |  |
Minimum |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Award waiting period |  |  | 2 years |  |  |
Maximum |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Award waiting period |  |  | 4 years |  |  |
Common Stock Buyback |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Remaining stock repurchase authorization amount |  |  | $ 282,079,000 |  |  |
2014 Equity Compensation Plan | Tranche One |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Vesting rate |  |  | 50.00% |  |  |
2014 Equity Compensation Plan | Tranche Two |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Vesting rate |  |  | 50.00% |  |  |
Employee Stock Purchase Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Employee stock purchase plan, employee eligible percentage on offerings of common stock |  |  | 85.00% |  |  |
Shares reserved under the plan (in shares) | shares |  |  | 15,652,000 |  |  |
Issuance of common stock under the employee stock purchase plan (in shares) | shares |  |  | 12,482,000 |  |  |
Employee stock purchase plan |  |  | $ 0 | $ 0 | $ 0 |

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