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Marketable Securities (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investments [Line Items]
Net unrealized holding gains	$ 1,193us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax	$ 1,386us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax
Unrealized holding gains during the period, income tax expense	703seic_AccumulatedOtherComprehensiveIncomeLossAvailableforsaleSecuritiesAdjustmentTax	430seic_AccumulatedOtherComprehensiveIncomeLossAvailableforsaleSecuritiesAdjustmentTax
Available-for-sale securities, gross realized gains	1,401us-gaap_AvailableForSaleSecuritiesGrossRealizedGains	1,236us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Available-for-sale securities, gross realized losses	448us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses	772us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Investment funds sponsored by LSV	4,523us-gaap_OtherMarketableSecuritiesNoncurrent	4,849us-gaap_OtherMarketableSecuritiesNoncurrent
Securities owned	21,175us-gaap_FinancialInstrumentsOwnedUSGovernmentAndAgencyObligationsAtFairValue	21,133us-gaap_FinancialInstrumentsOwnedUSGovernmentAndAgencyObligationsAtFairValue
Investment funds sponsored by LSV [Member]
Schedule of Investments [Line Items]
Investment funds sponsored by LSV	4,523us-gaap_OtherMarketableSecuritiesNoncurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsForeignMember	4,849us-gaap_OtherMarketableSecuritiesNoncurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsForeignMember
Change in fair value of investment funds sponsored by LSV	$ (326)us-gaap_RealizedInvestmentGainsLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsForeignMember	$ 143us-gaap_RealizedInvestmentGainsLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsForeignMember	$ 1,004us-gaap_RealizedInvestmentGainsLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsForeignMember

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Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

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The fair value as of the balance sheet date of firm holdings in debt obligations issued by the US government including short-term Treasury bills, medium-term Treasury notes, and long-term Treasury bonds, as well as debt issued by agencies. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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