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Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 167,821	$ 143,126
Securities owned	31,242	29,583
Total financial assets measured at fair value	369,988
Total financial liabilities measured at fair value	109,718
SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned	31,242	29,583
Total financial assets measured at fair value	263,944	221,368
Contingent considerations	9,252	14,355
Total financial liabilities measured at fair value	9,252	14,355
Consolidated VIEs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets measured at fair value	106,044
Securities sold short	100,466
Total financial liabilities measured at fair value	100,466
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets measured at fair value	159,538
Total financial liabilities measured at fair value	100,466
Level 1 | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned	0	0
Total financial assets measured at fair value	53,494	40,530
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]
Total financial assets measured at fair value	106,044
Securities sold short	100,466
Total financial liabilities measured at fair value	100,466
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets measured at fair value	199,063
Total financial liabilities measured at fair value	0
Level 2 | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned	31,242	29,583
Total financial assets measured at fair value	199,063	172,709
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]
Total financial assets measured at fair value	0
Securities sold short	0
Total financial liabilities measured at fair value	0
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets measured at fair value	0
Total financial liabilities measured at fair value	9,252
Level 3 | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned	0	0
Total financial assets measured at fair value	0	0
Contingent considerations	9,252	14,355
Total financial liabilities measured at fair value	9,252	14,355
Level 3 | Consolidated VIEs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets measured at fair value	0
Securities sold short	0
Total financial liabilities measured at fair value	0
NAV as a Practical Expedient
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets measured at fair value	11,387
Total financial liabilities measured at fair value	0
NAV as a Practical Expedient | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned	0	0
Total financial assets measured at fair value	11,387	8,129
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]
Total financial assets measured at fair value	0
Securities sold short	0
Total financial liabilities measured at fair value	0
Equity securities | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	53,494	40,530
Equity securities | Consolidated VIEs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	106,044
Equity securities | Level 1 | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	53,494	40,530
Equity securities | Level 1 | Consolidated VIEs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	106,044
Equity securities | Level 2 | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	0	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	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	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	167,821	143,126
Available-for-sale debt securities | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	167,821	143,126
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	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	167,821	143,126
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	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	0
Investment funds sponsored by LSV | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments measured at NAV	10,522	8,129
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	10,522	$ 8,129
Investments in limited partnership funds | SEI Investments Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments measured at NAV	865
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
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
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
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	$ 865

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- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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Fair value portion of asset recognized for present right to economic benefit.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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