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Acquisitions (Schedule of Intangible Assets Acquired as Part of Business Combination) (Details) | 12 Months Ended
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Dec. 31, 2014
Contractor relations | Maximum [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 5 years
Contractor relations | Minimum [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 2 years
Customer Relationships [Member] | Maximum [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 10 years
Customer Relationships [Member] | Minimum [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 2 years
Non-compete agreements | Maximum [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 7 years
Non-compete agreements | Minimum [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 3 years
Computer Software, Intangible Asset [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 6 years
Trademarks [Member] |
Acquired Intangible Assets [Line Items] |
Useful life | 2 years

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- Definition Acquired Finite Lived Intangible Assets Average Useful Life + References No definition available. + Details
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Namespace Prefix: | asgn_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: | asgn_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_ComputerSoftwareIntangibleAssetMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_NoncompeteAgreementsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |