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Business Segment Information (Schedule Of Additional Information Pertaining To Business Segments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Capital Expenditures	$ 15,375us-gaap_CapitalExpendituresIncurredButNotYetPaid	$ 17,035us-gaap_CapitalExpendituresIncurredButNotYetPaid
Depreciation	5,995us-gaap_Depreciation	5,584us-gaap_Depreciation
Amortization	10,358us-gaap_AdjustmentForAmortization	9,214us-gaap_AdjustmentForAmortization
Private Banks [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	8,502us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	8,798us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember
Depreciation	3,097us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	3,732us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember
Amortization	7,251us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	5,952us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember
Investment Advisors [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	3,318us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	3,589us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember
Depreciation	838us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	491us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember
Amortization	2,400us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	2,197us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember
Institutional Investors [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	947us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	1,288us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember
Depreciation	303us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	209us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember
Amortization	375us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	341us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember
Investment Managers [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	2,048us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	2,135us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember
Depreciation	999us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	512us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember
Amortization	250us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	227us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember
Investments In New Businesses [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	151us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	580us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember
Depreciation	563us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	530us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember
Amortization	25us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	440us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember
Total From Business Segments [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	14,966us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	16,390us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember
Depreciation	5,800us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	5,474us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember
Amortization	10,301us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	9,157us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember
Corporate Overhead [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	409us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	645us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember
Depreciation	195us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	110us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember
Amortization	$ 57us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	$ 57us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember

X

- Definition

The aggregate amount of recurring noncash expense charged against earnings in the period to allocate the cost of assets over their estimated remaining economic lives.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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X

- Definition

Future cash outflow to pay for purchases of fixed assets that have occurred.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

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X

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

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