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Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2011
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 667,446us-gaap_CashAndCashEquivalentsAtCarryingValue	$ 578,273us-gaap_CashAndCashEquivalentsAtCarryingValue	$ 452,247us-gaap_CashAndCashEquivalentsAtCarryingValue	$ 667,446us-gaap_CashAndCashEquivalentsAtCarryingValue	$ 420,986us-gaap_CashAndCashEquivalentsAtCarryingValue
Restricted cash	5,801us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue	5,500us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue		5,801us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue
Net book value of the SEI Wealth Platform	309,040us-gaap_CapitalizedComputerSoftwareNet	312,615us-gaap_CapitalizedComputerSoftwareNet		309,040us-gaap_CapitalizedComputerSoftwareNet
Anti-dilutive employee stock options	10,166us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	7,736us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	13,202us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Anti-dilutive employee stock options, average exercise price per share	$ 30.00us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount	$ 30.54us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount	$ 24.76us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount
SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Net book value of the SEI Wealth Platform	309,040us-gaap_CapitalizedComputerSoftwareNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember			309,040us-gaap_CapitalizedComputerSoftwareNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember
Capitalized software development costs related to the SEI Wealth Platform	34,877us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember	39,500us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember	31,004us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember
Conversion option payment included in software development costs		8,812us-gaap_PaymentsToAcquireSoftware / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember
Estimated useful life of the SEI Wealth Platform				15 years
Amortization expense related to the SEI Wealth Platform	38,357us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember	34,045us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember	32,177us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember
SEI Wealth Platform [Member] | Amortization [Member]
Summary of Significant Accounting Policies [Line Items]
Capitalized computer software impairments			2,661us-gaap_CapitalizedComputerSoftwareImpairments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember / us-gaap_IncomeStatementLocationAxis = seic_AmortizationExpenseMember
Sei Sponsored Open Ended Money Market Mutual Funds [Member]
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents	435,268us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember	387,201us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember		435,268us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember
Sei Investments Europe Limited [Member]
Summary of Significant Accounting Policies [Line Items]
Restricted cash	5,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember	5,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember		5,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember
Sei Investments Distribution Co Sidco [Member]
Summary of Significant Accounting Policies [Line Items]
Restricted cash	$ 500us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsDistributionCoSidcoMember	$ 500us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsDistributionCoSidcoMember		$ 500us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsDistributionCoSidcoMember
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Equity method investment, ownership percentage	50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_RangeAxis = us-gaap_MaximumMember			50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_RangeAxis = us-gaap_MaximumMember
Weighted Average [Member] | SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform				7 years 6 months
Building and Building Improvements [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	25 years
Building and Building Improvements [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	39 years
Computer Equipment [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	3 years
Computer Equipment [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	5 years
Furniture and Fixtures [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	3 years
Furniture and Fixtures [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	5 years
Equity Method Investee [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Equity method investment, ownership percentage	20.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = us-gaap_EquityMethodInvesteeMember / us-gaap_RangeAxis = us-gaap_MinimumMember			20.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = us-gaap_EquityMethodInvesteeMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Equity Method Investee [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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