Business Segment Information (Schedule Of Financial Information About Business Segments) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 334,263	$ 325,444
Expenses	234,404	219,015
Operating profit (loss)	99,859	106,429
Gain on sale of subsidiary	2,791	2,791
Segment profit (loss)	102,650	109,220
Private Banks [Member]
Segment Reporting Information [Line Items]
Revenues	113,361	111,213
Expenses	103,741	99,256
Operating profit (loss)	9,620	11,957
Gain on sale of subsidiary	2,791	2,791
Segment profit (loss)	12,411	14,748
Investment Advisors [Member]
Segment Reporting Information [Line Items]
Revenues	76,679	74,015
Expenses	44,774	39,059
Operating profit (loss)	31,905	34,956
Gain on sale of subsidiary	0	0
Segment profit (loss)	31,905	34,956
Institutional Investors [Member]
Segment Reporting Information [Line Items]
Revenues	72,897	73,548
Expenses	35,382	35,211
Operating profit (loss)	37,515	38,337
Gain on sale of subsidiary	0	0
Segment profit (loss)	37,515	38,337
Investment Managers [Member]
Segment Reporting Information [Line Items]
Revenues	69,918	65,367
Expenses	45,275	40,623
Operating profit (loss)	24,643	24,744
Gain on sale of subsidiary	0	0
Segment profit (loss)	24,643	24,744
Investments In New Businesses [Member]
Segment Reporting Information [Line Items]
Revenues	1,408	1,301
Expenses	5,232	4,866
Operating profit (loss)	(3,824)	(3,565)
Gain on sale of subsidiary	0	0
Segment profit (loss)	$ (3,824)	$ (3,565)

Total profit (loss)

No definition available.

Name:	seic_SegmentProfitLoss
Namespace Prefix:	seic_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount before tax of gain (loss) recognized on the sale or disposal of a disposal group. Excludes discontinued operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2443-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230

+ Details

Name:	us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1,2)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_GrossProfit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Excludes Selling, General and Administrative Expense.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingCostsAndExpenses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=seic_PrivateBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=seic_InvestmentAdvisorsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=seic_InstitutionalInvestorsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=seic_InvestmentManagersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=seic_InvestmentsInNewBusinessesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: