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Investment In Unconsolidated Affiliates (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Oct. 31, 2012
Apr. 17, 2015
Investments in and Advances to Affiliates [Line Items]        
Distributions received from unconsolidated affiliate $ 37,003us-gaap_EquityMethodInvestmentDividendsOrDistributions $ 31,505us-gaap_EquityMethodInvestmentDividendsOrDistributions    
Company's share in the earnings of equity method investee 34,033us-gaap_IncomeLossFromEquityMethodInvestments 31,891us-gaap_IncomeLossFromEquityMethodInvestments    
LSV Employee Group III [Member]        
Investments in and Advances to Affiliates [Line Items]        
Purchase of partnership interest value     77,700seic_PurchaseOfPartnershipInterestValue
/ dei_LegalEntityAxis
= seic_LsvEmployeeGroupIiiMember
 
Amount of purchase price financed through term loans     69,930us-gaap_LoansPayable
/ dei_LegalEntityAxis
= seic_LsvEmployeeGroupIiiMember
 
Financial Guarantee [Member] | L S V Asset Management [Member]        
Investments in and Advances to Affiliates [Line Items]        
Amount of guarantor's obligation, maximum exposure     24,930us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ dei_LegalEntityAxis
= seic_LSVAssetManagementMember
 
Financial Guarantee [Member] | LSV Employee Group III [Member]        
Investments in and Advances to Affiliates [Line Items]        
Amount of guarantor's obligation, maximum exposure     45,000us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ dei_LegalEntityAxis
= seic_LsvEmployeeGroupIiiMember
 
Financial Guarantee [Member] | LSV Employee Group III [Member] | Subsequent Event [Member]        
Investments in and Advances to Affiliates [Line Items]        
Amount of guarantor's obligation, maximum exposure       33,979us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ dei_LegalEntityAxis
= seic_LsvEmployeeGroupIiiMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Amount of guarantor's obligation, current carrying value       0us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ dei_LegalEntityAxis
= seic_LsvEmployeeGroupIiiMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
L S V Asset Management [Member]        
Investments in and Advances to Affiliates [Line Items]        
Total investment in equity method investee 50,282us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= seic_LSVAssetManagementMember
     
Distributions received from unconsolidated affiliate 37,003us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= seic_LSVAssetManagementMember
31,505us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= seic_LSVAssetManagementMember
   
Company's share in the earnings of equity method investee $ 34,345us-gaap_IncomeLossFromEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= seic_LSVAssetManagementMember
$ 32,216us-gaap_IncomeLossFromEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= seic_LSVAssetManagementMember
   
Maximum [Member]        
Investments in and Advances to Affiliates [Line Items]        
Equity method investment, ownership percentage 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember