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Income Taxes (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Income Tax Contingency [Line Items] |  |  |
Gross liability for unrecognized tax benefits, exclusive of interest and penalties | $ 14,373 seic_GrossLiabilityForUnrecognizedTaxBenefitsExclusiveOfInterestAndPenalties |  | $ 14,018 seic_GrossLiabilityForUnrecognizedTaxBenefitsExclusiveOfInterestAndPenalties
Unrecognized tax benefits that would affect effective tax rate | 12,528 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |  | 12,162 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Interest and penalties on unrecognized benefits | 1,189 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued |  | 1,066 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
Effective tax rates | 35.40%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 35.90%us-gaap_EffectiveIncomeTaxRateContinuingOperations |
Settlement and Lapse of Statute [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Unrecognized tax benefits within the next twelve months | $ 4,771 us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = seic_SettlementandLapseofStatuteMember |  |

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- Definition Gross Liability For Unrecognized Tax Benefits Exclusive Of Interest And Penalties + References No definition available. + Details
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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Subparagraph 2 -Article 4 + Details
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- Definition The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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