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Marketable Securities (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Schedule of Investments [Line Items] |  |  |
Net unrealized gains | $ 1,106 us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax |  | $ 1,193 us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax
Unrealized gains during the period, income tax expense | 676 seic_AccumulatedOtherComprehensiveIncomeLossAvailableforsaleSecuritiesAdjustmentTax |  | 703 seic_AccumulatedOtherComprehensiveIncomeLossAvailableforsaleSecuritiesAdjustmentTax
Available-for-sale securities, gross realized gains | 288 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | 119 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains |
Available-for-sale securities, gross realized losses | 142 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses | 49 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |
Investment funds sponsored by LSV | 4,454 us-gaap_OtherMarketableSecuritiesNoncurrent |  | 4,523 us-gaap_OtherMarketableSecuritiesNoncurrent
Securities owned | 21,192 us-gaap_FinancialInstrumentsOwnedUSGovernmentAndAgencyObligationsAtFairValue |  | 21,175 us-gaap_FinancialInstrumentsOwnedUSGovernmentAndAgencyObligationsAtFairValue
Private Equity Funds, Foreign [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Investment funds sponsored by LSV | $ 4,454 us-gaap_OtherMarketableSecuritiesNoncurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsForeignMember |  | $ 4,523 us-gaap_OtherMarketableSecuritiesNoncurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsForeignMember

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- Definition Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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