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Business Segments Long-lived assets by Segment (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Segment Reporting, Asset Reconciling Item [Line Items] |  |
Long-Lived Assets | $ 73.7 | $ 79.1
Domestic [Member] |  |
Segment Reporting, Asset Reconciling Item [Line Items] |  |
Long-Lived Assets | 71.4 | 76.7
Foreign [Member] |  |
Segment Reporting, Asset Reconciling Item [Line Items] |  |
Long-Lived Assets | $ 2.3 | $ 2.4

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- Definition Long-lived assets other than financial instruments, long-term customer relationships of a financial institution, mortgage and other servicing rights, deferred policy acquisition costs, and deferred tax assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 41 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e9038-108599 + Details
Name: | us-gaap_NoncurrentAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_StatementGeographicalAxis=asgn_DomesticMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementGeographicalAxis=asgn_ForeignMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |