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Document And Entity Information (USD $)
In Billions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Jan. 30, 2015
Jun. 30, 2014
Document And Entity Information [Abstract]      
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-K    
Document Period End Date Dec. 31, 2014    
Document Fiscal Year Focus 2014    
Document Fiscal Period Focus FY    
Trading Symbol SEIC    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   166,832,044dei_EntityCommonStockSharesOutstanding  
Entity Public Float     $ 4.3dei_EntityPublicFloat
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes