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Shareholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands |  |  | 12 Months Ended
--- | --- | --- | ---
Dec. 08, 2020 $ / shares | Jun. 03, 2020 $ / shares | Dec. 31, 2020 USD ($) $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) $ / 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 |  |  | $ 13.53 | $ 13.94 | $ 13.04
Increase (decrease) in stock-based compensation from change in management's estimate |  |  | $ (2,659) | $ 2,903 | $ 1,909
Unvested employee stock options outstanding (in shares) | shares |  |  | 11,854,000 |  |
Unrecognized compensation cost from employee stock options |  |  | $ 112,593 |  |
Weighted average period for recognition of unrecognized compensation cost |  |  | 1 year 8 months 12 days |  |
Total options that were exercisable (in shares) | shares |  |  | 6,641,000 | 8,214,000 |
Options outstanding weighted average remaining contractual life, years |  |  | 6 years 10 months 24 days |  |
Total intrinsic value of options exercised |  |  | $ 46,426 | $ 74,163 |
Aggregate intrinsic value of options exercisable |  |  | 120,357 |  |
Total intrinsic value of options outstanding |  |  | $ 158,204 |  |
Share price of SEI common stock (in USD per share) | $ / shares |  |  | $ 57.47 |  |
Percentage of one share for each unit purchased in Rights Agreement |  |  | 0.020 |  |
Dividends declared per common share (in USD per share) | $ / shares | $ 0.37 | $ 0.35 | $ 0.72 | $ 0.68 | $ 0.63
Dividends declared |  |  | $ 104,588 | $ 102,435 | $ 97,900
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 |  |  | $ 192,827 |  |
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 | shares |  |  | 15,652,000 |  |
Issuance of common stock under the employee stock purchase plan, shares | shares |  |  | 12,225,000 |  |
Employee stock purchase plan |  |  | $ 0 | $ 0 | $ 0

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