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Sale of SEI Asset Korea (Narrative) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 28, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Gain on sale of subsidiary $ 2,791us-gaap_GainOrLossOnSaleOfStockInSubsidiary $ 5,582us-gaap_GainOrLossOnSaleOfStockInSubsidiary  
Gain on sale of subsidiary, diluted earnings per share impact, net $ 0.01us-gaap_IncomeLossFromExtraordinaryItemsNetOfTaxPerDilutedShare $ 0.02us-gaap_IncomeLossFromExtraordinaryItemsNetOfTaxPerDilutedShare  
SEI Asset Korea Co., Ltd. [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Contingent purchase price from sale of SEI AK     $ 11,220seic_DisposalGroupProceedsFromSaleContingentPurchaseAmount
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= seic_SeiAssetKoreaCoLtdMember