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Rights Agreement, Percentage Of One Share For Each Unit Purchased

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Securities and assets of equivalent value, purchase price, per unit

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

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

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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

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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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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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Amount of additional cost recognized for award under share-based payment arrangement from occurrence of event accelerating recognition of cost.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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)					12 Months Ended
Shareholders' Equity (Narrative) (Details)	Jan. 08, 2020 $ / shares	Dec. 09, 2019 $ / shares	Jun. 20, 2019 $ / shares	May 29, 2019 $ / shares	Dec. 31, 2019 USD ($) right / shares $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted \| $ / shares					$ 13.94	$ 13.04	$ 16.78
Increase in stock-based compensation from change in management estimate					$ 2,903,000	$ 1,909,000	$ 11,206,000
Unvested employee stock options outstanding \| shares					7,492,000
Unrecognized compensation cost from employee stock options					$ 73,337,000
Weighted average period for recognition of unrecognized compensation cost					1 year 10 months 24 days
Total options that were exercisable (in shares) \| shares					8,214,000	9,359,000
Options outstanding weighted average remaining contractual life, years					6 years 6 months
Total intrinsic value of options exercised					$ 74,163,000	$ 139,087,000
Aggregate intrinsic value of options exercisable					231,115,000
Total intrinsic value of options outstanding					$ 293,500,000
Share price of SEI common stock (in USD per share) \| $ / shares					$ 65.48
Rights received by common shareholder for each common share outstanding, rights \| right / shares					1
Percentage of one share for each unit purchased in Rights Agreement					0.020
Series A Junior Participating Preferred stock, par value (in USD per share) \| $ / shares					$ 0.05	$ 0.05
Securities and assets of equivalent value, purchase price (in USD per unit) \| $ / shares					150.00
Dividends declared per common share (in USD per share) \| $ / shares		$ 0.35		$ 0.33	$ 0.68	$ 0.63	$ 0.58
Dividends paid per common share (in USD per share) \| $ / shares			$ 0.33
Dividends declared					$ 102,435,000	$ 97,900,000	$ 91,444,000
Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividends paid per common share (in USD per share) \| $ / shares	$ 0.35
Common Stock Buyback
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining stock repurchase authorization amount					$ 117,530,000
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					15,600,000
Issuance of common stock under the employee stock purchase plan, shares \| shares					12,131,000
Employee stock purchase plan					$ 0	$ 0	$ 0
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