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Shareholders' Equity (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 0 Months Ended | 12 Months Ended | 0 Months Ended
--- | --- | --- | ---
Dec. 09, 2014 | Jun. 24, 2014 | May 21, 2014 | Dec. 28, 2012 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Jan. 06, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Weighted average fair value of stock options granted |  |  |  |  | $ 10.88 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 10.45 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 6.73 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue |
Change in management estimate of stock-based compensation expense amortization |  |  |  |  |  | $ 19,637 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost |  |
Unrecognized compensation cost |  |  |  |  | 45,115 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |  |
Weighted average period for recognition of unrecognized compensation cost |  |  |  |  | 2 years 5 months 12 days |  |  |
Total options that were exercisable |  |  |  |  | 10,295,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber | 14,601,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber | 9,760,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber |
Options outstanding weighted average remaining contractual life, years |  |  |  |  | 5 years 9 months 15 days |  |  |
Total intrinsic value of options exercised |  |  |  |  | 83,196 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | 47,392 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue |  |
Aggregate intrinsic value of options exercisable |  |  |  |  | 200,328 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 |  |  |
Total intrinsic value of options outstanding |  |  |  |  | 302,441 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue |  |  |
Share price of SEI common stock |  |  |  |  | $ 40.04 us-gaap_SharePrice |  |  |
Rights received by common shareholder for each common share outstanding, rights |  |  |  |  | 1 seic_RightsAgreementConversionRateofRightsPerCommonShare |  |  |
Series A Junior Participating Preferred stock, par value |  |  |  |  | $ 0.05 us-gaap_PreferredStockParOrStatedValuePerShare | $ 0.05 us-gaap_PreferredStockParOrStatedValuePerShare |  |
Securities and assets of equivalent value, purchase price, per unit |  |  |  |  | $ 150 seic_RightsAgreementSecuritiesPurchasePricePerUnit |  |  |
Number of days, rights become exercisable following public announcement |  |  |  |  | 0 years 0 months 10 days |  |  |
Acquiring Person, beneficial ownership of the outstanding common stock, minimum percentage |  |  |  |  | 20.00%seic_RightsAgreementTriggeringEventVotingInterest |  |  |
Common shares received by holders upon surrender of rights, percentage |  |  |  |  | 50.00%seic_RightsAgreementPercentageofRightsReceivedAfterSurrender |  |  |
Rights non-voting, redeemable days |  |  |  |  | 0 years 0 months 10 days |  |  |
Right redeemable price, per right |  |  |  |  | 0.01 seic_RightsAgreementPurchasePricePerRight |  |  |
Dividends declared per common share | $ 0.24 us-gaap_CommonStockDividendsPerShareDeclared |  | $ 0.22 us-gaap_CommonStockDividendsPerShareDeclared |  | $ 0.46 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.42 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.63 us-gaap_CommonStockDividendsPerShareDeclared |
Dividends paid per common share |  | $ 0.22 us-gaap_CommonStockDividendsPerShareCashPaid |  |  |  |  |  |
Dividends declared |  |  |  | 55,149 us-gaap_DividendsCommonStock | 77,158 us-gaap_DividendsCommonStock | 71,665 us-gaap_DividendsCommonStock | 108,817 us-gaap_DividendsCommonStock |
Dividends paid |  |  |  | 55,149 us-gaap_Dividends |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Dividends paid per common share |  |  |  |  |  |  |  | $ 0.24 us-gaap_CommonStockDividendsPerShareCashPaid / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Common Stock Buyback [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Remaining stock repurchase authorization amount |  |  |  |  | $ 102,713 us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1 / us-gaap_ShareRepurchaseProgramAxis = seic_CommonStockBuybackMember |  |  |
Minimum [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Expiration date for options outstanding |  |  |  |  | Dec. 14, 2015 |  |  |
Maximum [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Expiration date for options outstanding |  |  |  |  | Dec. 09, 2024 |  |  |
2014 Equity Compensation Plan [Member] | Tranche One [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Vesting rate |  |  |  |  | 50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage / us-gaap_PlanNameAxis = seic_TwoThousandFourteenPlanMember / us-gaap_VestingAxis = seic_TrancheOneMember |  |  |
2014 Equity Compensation Plan [Member] | Tranche Two [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Vesting rate |  |  |  |  | 50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage / us-gaap_PlanNameAxis = seic_TwoThousandFourteenPlanMember / us-gaap_VestingAxis = seic_TrancheTwoMember |  |  |
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%seic_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketPaidPriceOfferingDate / us-gaap_PlanNameAxis = seic_EmployeeStockPurchasePlanMember |  |  |
Shares reserved under the plan |  |  |  |  | 15,600,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = seic_EmployeeStockPurchasePlanMember |  |  |
Issuance of common stock under the employee stock purchase plan, shares |  |  |  |  | 11,732,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_PlanNameAxis = seic_EmployeeStockPurchasePlanMember |  |  |

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Percentage of vesting of share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Date the equity-based award expires, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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