Rights received by common shareholder for each common share outstanding, rights.

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Number of days, rights become exercisable following public announcement.

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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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Aggregate dividends declared 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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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the 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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Additional share-based compensation cost recognized as a result of an occurrence of an event that accelerates its recognition.

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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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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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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 share-based compensation awards.

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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
	Jan. 08, 2019 $ / shares	Dec. 11, 2018 $ / shares	Jun. 22, 2018 $ / shares	May 30, 2018 $ / shares	Dec. 31, 2018 USD ($) right / shares $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted \| $ / shares					$ 13.04	$ 16.78	$ 12.43
Increase in stock-based compensation from change in management estimate \| $					$ 1,909	$ 11,206
Unvested employee stock options outstanding \| shares					6,454,000
Unrecognized compensation cost from employee stock options \| $					$ 68,470
Weighted average period for recognition of unrecognized compensation cost					2 years 2 months 13 days
Total options that were exercisable (in shares) \| shares					9,359,000	10,624,000	6,692,000
Options outstanding weighted average remaining contractual life, years					6 years 3 months 26 days
Total intrinsic value of options exercised \| $					$ 139,087	$ 83,628
Aggregate intrinsic value of options exercisable \| $					138,706
Total intrinsic value of options outstanding \| $					$ 143,939
Share price of SEI common stock (in USD per share) \| $ / shares					$ 46.20
Rights received by common shareholder for each common share outstanding, rights \| right / shares					1
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
Number of days, rights become exercisable following public announcement					10 days
Dividends declared per common share (in USD per share) \| $ / shares		$ 0.33		$ 0.30	$ 0.63	$ 0.58	$ 0.54
Dividends paid per common share (in USD per share) \| $ / shares			$ 0.30
Dividends declared \| $					$ 97,900	$ 91,444	$ 86,657
Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividends paid per common share (in USD per share) \| $ / shares	$ 0.33
Common Stock Buyback
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining stock repurchase authorization amount \| $					$ 215,879
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					12,038,000