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Document And Entity Information
 9 Months Ended
Sep. 30, 2013
Oct. 22, 2013
Document Information [Line Items]    
Entity Registrant Name SEI INVESTMENTS CO  
Entity Central Index Key 0000350894  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Document Type 10-Q  
Document Period End Date Sep. 30, 2013  
Document Fiscal Year Focus 2013  
Document Fiscal Period Focus Q3  
Trading Symbol SEIC  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   170,608,053