Rights Agreement, Percentage Of One Share For Each Unit Purchased

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Share-Based Compensation Arrangement By Share-Based Payment Award, Award Waiting Period

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Share Based Compensation Arrangement By Share Based Payment Award, Market Paid Price Offering Date

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Share-Based Payment Arrangement, Expense, Increase (Decrease)

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Aggregate dividends declared during the period for each share of common stock outstanding.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Number of shares authorized for issuance under share-based payment arrangement.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Price of a single share of a number of saleable stocks of a company.

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Percentage of vesting of award under share-based payment arrangement.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Number of non-vested options outstanding.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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Amount remaining of a stock repurchase plan authorized.

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Shareholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Dec. 08, 2020 $ / shares	Jun. 03, 2020 $ / shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted (in USD per share) \| $ / shares			$ 13.53	$ 13.94	$ 13.04
Increase (decrease) in stock-based compensation from change in management's estimate			$ (2,659)	$ 2,903	$ 1,909
Unvested employee stock options outstanding (in shares) \| shares			11,854,000
Unrecognized compensation cost from employee stock options			$ 112,593
Weighted average period for recognition of unrecognized compensation cost			1 year 8 months 12 days
Total options that were exercisable (in shares) \| shares			6,641,000	8,214,000
Options outstanding weighted average remaining contractual life, years			6 years 10 months 24 days
Total intrinsic value of options exercised			$ 46,426	$ 74,163
Aggregate intrinsic value of options exercisable			120,357
Total intrinsic value of options outstanding			$ 158,204
Share price of SEI common stock (in USD per share) \| $ / shares			$ 57.47
Percentage of one share for each unit purchased in Rights Agreement			0.020
Dividends declared per common share (in USD per share) \| $ / shares	$ 0.37	$ 0.35	$ 0.72	$ 0.68	$ 0.63
Dividends declared			$ 104,588	$ 102,435	$ 97,900
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award waiting period			2 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award waiting period			4 years
Common Stock Buyback
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining stock repurchase authorization amount			$ 192,827
2014 Equity Compensation Plan \| Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			50.00%
2014 Equity Compensation Plan \| Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			50.00%
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 \| shares			15,652,000
Issuance of common stock under the employee stock purchase plan, shares \| shares			12,225,000
Employee stock purchase plan			$ 0	$ 0	$ 0