Business Segment Information (Schedule Of Additional Information Pertaining To Business Segments) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Capital Expenditures	$ 20,173	$ 18,786	$ 37,427	$ 37,284
Depreciation	7,422	7,138	14,753	14,260
Amortization	12,781	12,161	25,460	24,015
Operating Segments
Segment Reporting Information [Line Items]
Capital Expenditures	19,587	18,420	36,456	36,610
Depreciation	7,094	6,841	14,089	13,672
Amortization	12,725	12,104	25,346	23,900
Operating Segments \| Private Banks
Segment Reporting Information [Line Items]
Capital Expenditures	8,761	9,529	17,222	19,768
Depreciation	3,585	3,323	7,134	6,642
Amortization	7,217	6,747	14,358	13,374
Operating Segments \| Investment Advisors
Segment Reporting Information [Line Items]
Capital Expenditures	4,558	4,284	8,505	8,544
Depreciation	1,165	1,105	2,344	2,210
Amortization	2,550	2,401	5,073	4,758
Operating Segments \| Institutional Investors
Segment Reporting Information [Line Items]
Capital Expenditures	1,049	997	1,920	1,964
Depreciation	414	452	819	900
Amortization	427	427	860	854
Operating Segments \| Investment Managers
Segment Reporting Information [Line Items]
Capital Expenditures	4,875	3,370	8,224	5,890
Depreciation	1,820	1,806	3,591	3,615
Amortization	2,346	2,345	4,685	4,690
Operating Segments \| Investments In New Businesses
Segment Reporting Information [Line Items]
Capital Expenditures	344	240	585	444
Depreciation	110	155	201	305
Amortization	185	184	370	224
Corporate
Segment Reporting Information [Line Items]
Capital Expenditures	586	366	971	674
Depreciation	328	297	664	588
Amortization	$ 56	$ 57	$ 114	$ 115

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

+ Details

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

Name:	us-gaap_Depreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_CorporateNonSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: