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Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 643,260us-gaap_CashAndCashEquivalentsAtCarryingValue	$ 545,552us-gaap_CashAndCashEquivalentsAtCarryingValue	$ 667,446us-gaap_CashAndCashEquivalentsAtCarryingValue	$ 578,273us-gaap_CashAndCashEquivalentsAtCarryingValue
Restricted cash	5,805us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue		5,801us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue
Net book value of the SEI Wealth Platform	306,728us-gaap_CapitalizedComputerSoftwareNet		309,040us-gaap_CapitalizedComputerSoftwareNet
Anti-dilutive employee stock options	10,051us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	4,389us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Antidilutive securities excluded from computation of net income, per outstanding unit, amount	$ 30.02us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount	$ 32.40us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	500us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsDistributionCoSidcoMember		500us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsDistributionCoSidcoMember
SEI Investments (Europe) Limited [Member]
Restricted cash	5,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember		5,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents invested in SEI-sponsored money market funds	419,278us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember		435,268us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember
SEI Wealth Platform [Member]
Capitalized software development costs related to the SEI Wealth Platform	7,330us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember	9,408us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember
Net book value of the SEI Wealth Platform	298,739us-gaap_CapitalizedComputerSoftwareNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember
Estimated useful life of the SEI Wealth Platform	15 years
Amortization expense related to the SEI Wealth Platform	$ 10,301us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember	$ 9,110us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember
Weighted Average [Member] | SEI Wealth Platform [Member]
Estimated useful life of the SEI Wealth Platform	7 years 3 months

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