Fair Value Measurements (Fair Value Inputs, Liabilities, Quantitative Information) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Fair Value Inputs, Liabilities, Quantitative Information
Unamortized deferred loan costs	$ 21.0	$ 23.6
Write-off of Intangible Assets (Excluding Goodwill)	3.3
Level 1 Inputs
Fair Value Inputs, Liabilities, Quantitative Information
Long-term debt, fair value	1,000.0
Deferred Compensation Plan Assets	$ 10.8	$ 6.2

Write-off Intangible Asset

No definition available.

Name:	asgn_WriteoffIntangibleAsset
Namespace Prefix:	asgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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Period Type: