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Shareholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | ---
Jan. 08, 2019 $ / shares | Dec. 11, 2018 $ / shares | Jun. 22, 2018 $ / shares | May 30, 2018 $ / shares | Dec. 31, 2018 USD ($) right / shares $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Weighted average fair value of stock options granted | $ / shares |  |  |  |  | $ 13.04 | $ 16.78 | $ 12.43
Increase in stock-based compensation from change in management estimate | $ |  |  |  |  | $ 1,909 | $ 11,206 |
Unvested employee stock options outstanding | shares |  |  |  |  | 6,454,000 |  |
Unrecognized compensation cost from employee stock options | $ |  |  |  |  | $ 68,470 |  |
Weighted average period for recognition of unrecognized compensation cost |  |  |  |  | 2 years 2 months 13 days |  |
Total options that were exercisable (in shares) | shares |  |  |  |  | 9,359,000 | 10,624,000 | 6,692,000
Options outstanding weighted average remaining contractual life, years |  |  |  |  | 6 years 3 months 26 days |  |
Total intrinsic value of options exercised | $ |  |  |  |  | $ 139,087 | $ 83,628 |
Aggregate intrinsic value of options exercisable | $ |  |  |  |  | 138,706 |  |
Total intrinsic value of options outstanding | $ |  |  |  |  | $ 143,939 |  |
Share price of SEI common stock (in USD per share) | $ / shares |  |  |  |  | $ 46.20 |  |
Rights received by common shareholder for each common share outstanding, rights | right / shares |  |  |  |  | 1 |  |
Series A Junior Participating Preferred stock, par value (in USD per share) | $ / shares |  |  |  |  | $ 0.05 | $ 0.05 |
Securities and assets of equivalent value, purchase price (in USD per unit) | $ / shares |  |  |  |  | $ 150.00 |  |
Number of days, rights become exercisable following public announcement |  |  |  |  | 10 days |  |
Dividends declared per common share (in USD per share) | $ / shares |  | $ 0.33 |  | $ 0.30 | $ 0.63 | $ 0.58 | $ 0.54
Dividends paid per common share (in USD per share) | $ / shares |  |  | $ 0.30 |  |  |  |
Dividends declared | $ |  |  |  |  | $ 97,900 | $ 91,444 | $ 86,657
Subsequent Event |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Dividends paid per common share (in USD per share) | $ / shares | $ 0.33 |  |  |  |  |  |
Common Stock Buyback |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Remaining stock repurchase authorization amount | $ |  |  |  |  | $ 215,879 |  |
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 |  |  |  |  | 15,600,000 |  |
Issuance of common stock under the employee stock purchase plan, shares | shares |  |  |  |  | 12,038,000 |  |

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