Business Segment Information (Schedule Of Financial Information About Business Segments) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 335,622	$ 322,047	$ 998,811	$ 943,248
Expenses	235,748	218,559	683,819	644,415
Operating profit (loss)	99,874	103,488	314,992	298,833
Gain on sale of subsidiary	0	0	2,791	5,582
Segment profit (loss)			317,783	304,415
Private Banks [Member]
Segment Reporting Information [Line Items]
Revenues	116,280	110,147	342,826	329,508
Expenses	104,217	100,498	308,200	299,645
Operating profit (loss)	12,063	9,649	34,626	29,863
Gain on sale of subsidiary	0	0	2,791	5,582
Segment profit (loss)			37,417	35,445
Investment Advisors [Member]
Segment Reporting Information [Line Items]
Revenues	76,238	74,469	228,006	210,939
Expenses	45,530	37,796	125,446	108,052
Operating profit (loss)	30,708	36,673	102,560	102,887
Gain on sale of subsidiary	0	0	0	0
Segment profit (loss)			102,560	102,887
Institutional Investors [Member]
Segment Reporting Information [Line Items]
Revenues	74,515	72,560	224,043	212,211
Expenses	36,923	35,495	108,662	104,615
Operating profit (loss)	37,592	37,065	115,381	107,596
Gain on sale of subsidiary	0	0	0	0
Segment profit (loss)			115,381	107,596
Investment Managers [Member]
Segment Reporting Information [Line Items]
Revenues	67,162	63,640	199,809	187,099
Expenses	43,899	40,156	126,663	118,789
Operating profit (loss)	23,263	23,484	73,146	68,310
Gain on sale of subsidiary	0	0	0	0
Segment profit (loss)			73,146	68,310
Investments In New Businesses [Member]
Segment Reporting Information [Line Items]
Revenues	1,427	1,231	4,127	3,491
Expenses	5,179	4,614	14,848	13,314
Operating profit (loss)	(3,752)	(3,383)	(10,721)	(9,823)
Gain on sale of subsidiary	$ 0	$ 0	0	0
Segment profit (loss)			$ (10,721)	$ (9,823)

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=51824106&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=51719941&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=26872669&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=26872669&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: