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

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- Definition Accumulated Other Comprehensive Income (Loss) Available For Sale Securities Adjustment Tax + References No definition available. + Details
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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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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969 + Details
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- Definition All other debt and equity financial instruments, which aren't categorized as available-for-sale, or held-to-maturity and which are not otherwise listed in the existing taxonomy, intended to be held longer than one year from the balance sheet date or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 325 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6485959&loc=d3e22529-158502 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 3 -Subparagraph a -Article 7 + Details
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