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Business Segment Information (Schedule Of Additional Information Pertaining To Business Segments) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Capital Expenditures	$ 63,346us-gaap_CapitalExpendituresIncurredButNotYetPaid	$ 55,851us-gaap_CapitalExpendituresIncurredButNotYetPaid	$ 54,074us-gaap_CapitalExpendituresIncurredButNotYetPaid
Depreciation	22,448us-gaap_Depreciation	22,497us-gaap_Depreciation	22,586us-gaap_Depreciation
Amortization	38,679us-gaap_AdjustmentForAmortization	34,602us-gaap_AdjustmentForAmortization	33,258us-gaap_AdjustmentForAmortization
Assets	1,542,875us-gaap_Assets	1,439,169us-gaap_Assets
Private Banks [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	30,883us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	34,258us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	32,509us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember
Depreciation	13,393us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	15,506us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	15,226us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember
Amortization	24,993us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	22,379us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	22,218us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember
Assets	417,890us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember	430,613us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_PrivateBankMember
Investment Advisors [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	13,783us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	12,611us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	11,193us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember
Depreciation	2,507us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	2,091us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	1,981us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember
Amortization	9,228us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	8,234us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	7,167us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember
Assets	134,371us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember	124,028us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentAdvisorsMember
Institutional Investors [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	4,575us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	2,712us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	2,781us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember
Depreciation	1,041us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	893us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	1,012us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember
Amortization	1,430us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	1,274us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	1,208us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember
Assets	118,397us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember	97,816us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InstitutionalInvestorsMember
Investment Managers [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	9,505us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	4,871us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	5,494us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember
Depreciation	2,917us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	1,970us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	1,914us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember
Amortization	954us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	851us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	804us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember
Assets	134,614us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember	122,969us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentManagersMember
Investments In New Businesses [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	2,547us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	639us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	632us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember
Depreciation	1,983us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	1,589us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	1,716us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember
Amortization	1,846us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	1,636us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	1,249us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember
Assets	21,830us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember	6,064us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_InvestmentsInNewBusinessesMember
Total From Business Segments [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	61,293us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	55,091us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	52,609us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember
Depreciation	21,841us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	22,049us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	21,849us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember
Amortization	38,451us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	34,374us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	32,646us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember
Assets	827,102us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember	781,490us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = seic_TotalFromBusinessSegmentsMember
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	2,053us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	760us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	1,465us-gaap_CapitalExpendituresIncurredButNotYetPaid / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember
Depreciation	607us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	448us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	737us-gaap_Depreciation / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember
Amortization	228us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	228us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	612us-gaap_AdjustmentForAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember
Assets	$ 715,773us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember	$ 657,679us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateNonSegmentMember

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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.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

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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.

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