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Composition of Certain Financial Statement Captions (Property And Equipment) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross $ 372,736us-gaap_PropertyPlantAndEquipmentGross   $ 366,830us-gaap_PropertyPlantAndEquipmentGross
Less: Accumulated depreciation (246,881)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment   (241,295)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Property and Equipment, net 125,855us-gaap_PropertyPlantAndEquipmentNet   125,535us-gaap_PropertyPlantAndEquipmentNet
Depreciation expense 5,995us-gaap_DepreciationNonproduction 5,584us-gaap_DepreciationNonproduction  
Building [Member]      
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross 150,644us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingMember
  149,890us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingMember
Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross 82,860us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_EquipmentMember
  78,266us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_EquipmentMember
Land [Member]      
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross 9,997us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
  9,997us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
Purchased Software [Member]      
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross 105,728us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= seic_PurchasedSoftwareMember
  104,964us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= seic_PurchasedSoftwareMember
Furniture and Fixtures [Member]      
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross 16,883us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_FurnitureAndFixturesMember
  16,944us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_FurnitureAndFixturesMember
Leasehold Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross 5,974us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LeaseholdImprovementsMember
  5,675us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LeaseholdImprovementsMember
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property and Equipment, gross $ 650us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
  $ 1,094us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember