Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2024	Jun. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 29, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted (in USD per share)			$ 23.07	$ 15.70	$ 17.95
(Decrease) increase in stock-based compensation from change in management's estimate			$ 11,181,000	$ (6,941,000)	$ (4,915,000)
Unvested employee stock options outstanding (in shares)	6,755		6,755
Unrecognized compensation cost from employee stock options	$ 67,910,000		$ 67,910,000
Weighted average period for recognition of unrecognized compensation cost			2 years 1 month 6 days
Options outstanding weighted average remaining contractual life, years			6 years 2 months 12 days
Total intrinsic value of options exercised			$ 54,101,000	$ 34,499,000
Aggregate intrinsic value of options exercisable	203,313,000		203,313,000
Total intrinsic value of options outstanding	$ 324,138,000		$ 324,138,000
Share price of SEI common stock (in USD per share)						$ 82.48
Dividends declared per common share (in USD per share)	$ 0.49	$ 0.46	$ 0.95	$ 0.89	$ 0.83
Cash dividends declared			$ (123,119,000)	$ (117,894,000)	$ (112,429,000)
Common Stock Buyback
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining stock repurchase authorization amount	$ 169,600,000		$ 169,600,000
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee stock purchase plan, employee eligible percentage on offerings of common stock			85.00%
Shares reserved under the plan (in shares)	15,652		15,652
Issuance of common stock under the employee stock purchase plan (in shares)			12,558
Employee stock purchase plan			$ 0	$ 0	$ 0
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for recognition of unrecognized compensation cost			1 year 10 months 24 days
Unrecognized compensation cost from employee stock options	$ 52,685,000		$ 52,685,000