v3.26.3
Fair Value Measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale securities $ 155,995 $ 158,690
Securities owned 25,750 33,777
Total financial assets measured at fair value 402,640 385,751
Total financial liabilities measured at fair value 139,242 116,077
SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   67,414
Available-for-sale securities   158,690
Securities owned 25,750 33,777
Total financial assets measured at fair value 267,372 272,632
Contingent considerations 7,834 7,834
Total financial liabilities measured at fair value 7,834 7,834
Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   113,119
Total financial assets measured at fair value 135,268 113,119
Securities sold short 131,408 108,243
Total financial liabilities measured at fair value 131,408 108,243
Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total financial assets measured at fair value 205,412 180,533
Total financial liabilities measured at fair value 131,408 108,243
Level 1 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   67,414
Available-for-sale securities   0
Securities owned 0 0
Total financial assets measured at fair value 70,144 67,414
Contingent considerations 0 0
Total financial liabilities measured at fair value 0 0
Level 1 | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   113,119
Total financial assets measured at fair value 135,268 113,119
Securities sold short 131,408 108,243
Total financial liabilities measured at fair value 131,408 108,243
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total financial assets measured at fair value 181,745 192,467
Total financial liabilities measured at fair value 0 0
Level 2 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   0
Available-for-sale securities   158,690
Securities owned 25,750 33,777
Total financial assets measured at fair value 181,745 192,467
Contingent considerations 0 0
Total financial liabilities measured at fair value 0 0
Level 2 | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   0
Total financial assets measured at fair value 0 0
Securities sold short 0 0
Total financial liabilities measured at fair value 0 0
Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total financial assets measured at fair value 0 0
Total financial liabilities measured at fair value 7,834 7,834
Level 3 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   0
Available-for-sale securities   0
Securities owned 0 0
Total financial assets measured at fair value 0 0
Contingent considerations 7,834 7,834
Total financial liabilities measured at fair value 7,834 7,834
Level 3 | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   0
Total financial assets measured at fair value 0 0
Securities sold short 0 0
Total financial liabilities measured at fair value 0 0
NAV as a Practical Expedient    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total financial assets measured at fair value 15,483 12,751
Total financial liabilities measured at fair value 0 0
NAV as a Practical Expedient | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   0
Available-for-sale securities   0
Securities owned 0 0
Total financial assets measured at fair value 15,483 12,751
Contingent considerations 0 0
Total financial liabilities measured at fair value 0 0
NAV as a Practical Expedient | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities   0
Total financial assets measured at fair value 0 0
Securities sold short 0 0
Total financial liabilities measured at fair value 0 0
Equity securities | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 70,144  
Equity securities | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 135,268  
Equity securities | Level 1 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 70,144  
Equity securities | Level 1 | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 135,268  
Equity securities | Level 2 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 0  
Equity securities | Level 2 | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 0  
Equity securities | Level 3 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 0  
Equity securities | Level 3 | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 0  
Equity securities | NAV as a Practical Expedient | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 0  
Equity securities | NAV as a Practical Expedient | Consolidated VIEs    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities 0  
Available-for-sale debt securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale securities 128,000 158,690
Available-for-sale debt securities | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale securities 155,995  
Available-for-sale debt securities | Level 1 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale securities 0  
Available-for-sale debt securities | Level 2 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale securities 155,995  
Available-for-sale debt securities | Level 3 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale securities 0  
Available-for-sale debt securities | NAV as a Practical Expedient | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale securities 0  
Investment funds sponsored by LSV | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 14,176 11,593
Investment funds sponsored by LSV | Level 1 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 0 0
Investment funds sponsored by LSV | Level 2 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 0 0
Investment funds sponsored by LSV | Level 3 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 0 0
Investment funds sponsored by LSV | NAV as a Practical Expedient | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 14,176 11,593
Investments in limited partnership funds | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 1,307 1,158
Investments in limited partnership funds | Level 1 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 0 0
Investments in limited partnership funds | Level 2 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 0 0
Investments in limited partnership funds | Level 3 | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV 0 0
Investments in limited partnership funds | NAV as a Practical Expedient | SEI Investments Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments measured at NAV $ 1,307 $ 1,158