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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 | $ 667,446 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 578,273 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 452,247 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 667,446 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 420,986 us-gaap_CashAndCashEquivalentsAtCarryingValue
Restricted cash | 5,801 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue | 5,500 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue |  | 5,801 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue |
Net book value of the SEI Wealth Platform | 309,040 us-gaap_CapitalizedComputerSoftwareNet | 312,615 us-gaap_CapitalizedComputerSoftwareNet |  | 309,040 us-gaap_CapitalizedComputerSoftwareNet |
Anti-dilutive employee stock options | 10,166 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 7,736 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 13,202 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount |  |
Anti-dilutive employee stock options, average exercise price per share | $ 30.00 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount | $ 30.54 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount | $ 24.76 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfNetIncomePerOutstandingUnitAmount |  |
SEI Wealth Platform [Member] |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Net book value of the SEI Wealth Platform | 309,040 us-gaap_CapitalizedComputerSoftwareNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember |  |  | 309,040 us-gaap_CapitalizedComputerSoftwareNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember |
Capitalized software development costs related to the SEI Wealth Platform | 34,877 us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember | 39,500 us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember | 31,004 us-gaap_CapitalizedComputerSoftwareAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember |  |
Conversion option payment included in software development costs |  | 8,812 us-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 | 38,357 us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember | 34,045 us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember | 32,177 us-gaap_CapitalizedComputerSoftwareAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = seic_SeiWealthPlatformMember |  |
SEI Wealth Platform [Member] | Amortization [Member] |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized computer software impairments |  |  | 2,661 us-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 | 435,268 us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember | 387,201 us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember |  | 435,268 us-gaap_CashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiSponsoredOpenEndedMoneyMarketMutualFundsMember |
Sei Investments Europe Limited [Member] |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Restricted cash | 5,000 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember | 5,000 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember |  | 5,000 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsEuropeLimitedMember |
Sei Investments Distribution Co Sidco [Member] |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Restricted cash | $ 500 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsDistributionCoSidcoMember | $ 500 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis = seic_SeiInvestmentsDistributionCoSidcoMember |  | $ 500 us-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] |  |  |  |  |
Equity method investment, ownership percentage | 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = us-gaap_EquityMethodInvesteeMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  | 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = us-gaap_EquityMethodInvesteeMember / us-gaap_RangeAxis = us-gaap_MaximumMember |

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