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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	$ 14,373seic_GrossLiabilityForUnrecognizedTaxBenefitsExclusiveOfInterestAndPenalties		$ 14,018seic_GrossLiabilityForUnrecognizedTaxBenefitsExclusiveOfInterestAndPenalties
Unrecognized tax benefits that would affect effective tax rate	12,528us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate		12,162us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Interest and penalties on unrecognized benefits	1,189us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued		1,066us-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	$ 4,771us-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.

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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

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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

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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)

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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

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