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Summary of Significant Accounting Policies - Company's Income from Investments Held in Employee Deferred Compensation Trusts (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Accounting Policies [Abstract] |  |
Dividend income | $ (294) | $ (405)
Realized and unrealized (gains) losses | (11,694) | 40,781
(Income) loss from investments held in employee deferred compensation trusts | $ (11,988) | $ 40,376

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- Definition Deferred Compensation Plan, Dividend Income + References No definition available. + Details
Name: | rhi_DeferredCompensationPlanDividendIncome
Namespace Prefix: | rhi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Deferred Compensation Plan, Investment Income + References No definition available. + Details
Name: | rhi_DeferredCompensationPlanInvestmentIncome
Namespace Prefix: | rhi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Deferred Compensation Plan, Realized and Unrealized Gain (Loss) + References No definition available. + Details
Name: | rhi_DeferredCompensationPlanRealizedAndUnrealizedGainLoss
Namespace Prefix: | rhi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration