Acquisitions - Additional Information (Details) - USD ($) $ in Millions		9 Months Ended
Mar. 04, 2025	Sep. 30, 2025
Intangible Assets [Line Items]
Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Indefinite-Lived	$ 32.4
Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Excluding Goodwill	78.9
Goodwill, Acquired During Period		$ 247.9
Customer and Contractual Relationships
Intangible Assets [Line Items]
Acquired Finite-Lived Intangible Assets, Useful Life		7 years
Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Finite-Lived	42.1
In-use software
Intangible Assets [Line Items]
Acquired Finite-Lived Intangible Assets, Useful Life		3 years
Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Finite-Lived	4.4
TopBloc [Member]
Intangible Assets [Line Items]
Business Combination, Price of Acquisition, Expected	$ 340.0
Business Combination, Price of Acquisition, Expected Cash Portion, Percentage [Member]	90.00%
Business Combination, Price of Acquisition, Expected Equity Portion, Percentage	10.00%
Business Combination, Goodwill, Expected Tax Deductible, Amount	$ 217.4
2024 Acquisitions [Member] \| Commercial
Intangible Assets [Line Items]
Goodwill, Acquired During Period		$ 247.9

Business Combination, Price of Acquisition, Expected Cash Portion, Percentage

No definition available.

Name:	asgn_BusinessCombinationPriceOfAcquisitionExpectedCashPortionPercentage
Namespace Prefix:	asgn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Business Combination, Price of Acquisition, Expected Equity Portion, Percentage

+ References

No definition available.

+ Details

Name:	asgn_BusinessCombinationPriceOfAcquisitionExpectedEquityPortionPercentage
Namespace Prefix:	asgn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Intangible Assets [Line Items]

+ References

No definition available.

+ Details

Name:	asgn_IntangibleAssetsLineItems
Namespace Prefix:	asgn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of goodwill arising from a business combination that is expected to be deductible for tax purposes.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-8

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-40

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-1

+ Details

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

- Definition

Purchase price of expected business acquisition prior to consideration being transferred. Excludes asset acquisition.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-7

+ Details

Name:	us-gaap_BusinessCombinationPriceOfAcquisitionExpected
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of identifiable indefinite-lived intangible asset acquired in business combination and recognized at acquisition date. Excludes goodwill and financial asset.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479907/805-20-50-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479907/805-20-50-1

+ Details

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

- Definition

Amount of identifiable intangible asset acquired in business combination and recognized at acquisition date. Excludes goodwill and financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-8

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 41
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-41

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 45
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-45

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479907/805-20-50-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479907/805-20-50-1

+ Details

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

- Definition

Amount of identifiable finite-lived intangible asset acquired in business combination and recognized at acquisition date.

+ References

+ Details

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

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 24
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482548/350-20-55-24

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-4

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-1

+ Details

Name:	us-gaap_GoodwillAcquiredDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=asgn_TopBlocMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=asgn_A2025AcquisitionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=asgn_CommercialBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: