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Shareholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | ---
Jan. 08, 2018 $ / shares | Dec. 12, 2017 $ / shares | Jun. 16, 2017 $ / shares | May 24, 2017 $ / shares | Dec. 31, 2017 USD ($) right / shares $ / right $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Weighted average fair value of stock options granted | $ / shares |  |  |  |  | $ 16.78 | $ 12.43 | $ 12.16
Increase in stock-based compensation from change in management estimate | $ |  |  |  |  | $ 11,206 |  |
Unvested employee stock options outstanding | shares |  |  |  |  | 6,088,000 |  |
Unrecognized compensation cost from employee stock options | $ |  |  |  |  | $ 62,070 |  |
Weighted average period for recognition of unrecognized compensation cost |  |  |  |  | 2 years 3 months 18 days |  |
Total options that were exercisable | shares |  |  |  |  | 10,624,000 | 6,692,000 | 8,508,000
Options outstanding weighted average remaining contractual life, years |  |  |  |  | 6 years 3 months 18 days |  |
Total intrinsic value of options exercised | $ |  |  |  |  | $ 83,628 | $ 41,607 |
Aggregate intrinsic value of options exercisable | $ |  |  |  |  | 455,528 |  |
Total intrinsic value of options outstanding | $ |  |  |  |  | $ 572,024 |  |
Share price of SEI common stock | $ / shares |  |  |  |  | $ 71.86 |  |
Rights received by common shareholder for each common share outstanding, rights | right / shares |  |  |  |  | 1 |  |
Series A Junior Participating Preferred stock, par value | $ / shares |  |  |  |  | $ 0.05 | $ 0.05 |
Securities and assets of equivalent value, purchase price, per unit | $ / shares |  |  |  |  | $ 150.00 |  |
Number of days, rights become exercisable following public announcement |  |  |  |  | 10 days |  |
Acquiring person, beneficial ownership of the outstanding common stock, minimum percentage |  |  |  |  | 20.00% |  |
Common shares received by holders upon surrender of rights, percentage |  |  |  |  | 50.00% |  |
Rights non-voting, redeemable days |  |  |  |  | 10 days |  |
Right redeemable price, per right | $ / right |  |  |  |  | 0.01 |  |
Dividends declared per common share | $ / shares |  | $ 0.30 |  | $ 0.28 | $ 0.58 | $ 0.54 | $ 0.50
Dividends paid per common share | $ / shares |  |  | $ 0.28 |  |  |  |
Dividends declared | $ |  |  |  |  | $ 91,444 | $ 86,657 | $ 82,478
Subsequent Event [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Dividends paid per common share | $ / shares | $ 0.30 |  |  |  |  |  |
Common Stock Buyback [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Remaining stock repurchase authorization amount | $ |  |  |  |  | $ 170,638 |  |
Minimum [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Expiration date for options outstanding |  |  |  |  | Dec. 16, 2018 |  |
Maximum [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Expiration date for options outstanding |  |  |  |  | Dec. 12, 2027 |  |
2014 Equity Compensation Plan [Member] | Tranche One [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Vesting rate |  |  |  |  | 50.00% |  |
2014 Equity Compensation Plan [Member] | Tranche Two [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Vesting rate |  |  |  |  | 50.00% |  |
Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |
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 |  |  |  |  | 11,962,000 |  |

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