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v3.20.2

Commitments and Contingencies (Details) $ in Millions | Sep. 30, 2020 USD ($) property | Dec. 31, 2019 USD ($)
--- | --- | ---
Other Commitments [Line Items] |  |
Deferred Compensation Liability, Classified, Noncurrent | $ | $ 13.1 | $ 11.8
Number of Leased Properties Owned by Related Parties | property | 2 |

X
- Definition Number of Leased Properties Owned by Related Parties + References No definition available. + Details
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Namespace Prefix: | asgn_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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