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Acquisitions (Schedule of Purchase Price Allocation) (Details) (USD $) | 0 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 0 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 02, 2013 | Dec. 31, 2013 | Dec. 31, 2013 | Dec. 31, 2013 | May 15, 2012 | Sep. 30, 2012 | Dec. 31, 2012 | Dec. 31, 2014 | Dec. 05, 2013
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
contingent consideration maximum |  |  |  |  |  |  |  | $ 16,000,000asgn_Contingentconsiderationmaximum |
CyberCoders [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets |  |  |  |  |  |  |  |  | 915,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets |  |  |  |  |  |  |  |  | 11,329,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  |  |  |  |  | 37,860,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Acquisition, Effective Date of Acquisition |  | Dec. 05, 2013 |  |  |  |  |  |  |
Business Acquisition, Name of Acquired Entity |  | CyberCoders Holdings, Inc |  |  |  |  |  |  |
Business Acquisition, Description of Acquired Entity |  | a provider of permanent placement services headquartered in Irvine, California. |  |  |  |  |  |  |
Business Combination, Reason for Business Combination |  | The primary reason for the acquisition was to expand the Company's permanent placement services. |  |  |  |  |  |  |
Business Combination, Consideration Transferred |  | 96,600,000us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment |  |  |  |  |  |  |  |  | 3,327,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
BusinessCombinationRecognizedGoodwill |  |  |  |  |  |  |  |  | 69,018,000asgn_BusinessCombinationRecognizedGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Goodwill, Acquired During Period |  |  | 69,018,000us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets |  |  |  |  |  |  |  |  | 122,449,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities |  |  |  |  |  |  |  |  | 9,022,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other |  |  |  |  |  |  |  |  | 16,839,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities |  |  |  |  |  |  |  |  | 25,861,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net |  |  |  |  |  |  |  |  | 96,588,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Acquisition, Cost of Acquired Entity, Cash Paid |  | 93,600,000us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember |  |  |  |  |  |  |
Business Acquisition Contingent Consideration At Fair Value Accrued Earn-Out | 700,000asgn_BusinessAcquisitionContingentConsiderationAtFairValueAccruedEarnOut / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember | 3,000,000asgn_BusinessAcquisitionContingentConsiderationAtFairValueAccruedEarnOut / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember |  |  |  |  |  |  |
contingent consideration maximum |  |  |  |  |  |  |  |  | 11,000,000asgn_Contingentconsiderationmaximum / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember
Business Combination, Acquisition Related Costs |  |  | 1,500,000us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember |  |  |  |  |  |
Whitaker [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets | 568,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets | 9,155,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill | 7,750,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  |  |  | Dec. 02, 2013 |  |  |  |  |
Business Acquisition, Name of Acquired Entity |  |  |  | Whitaker Medical, LLC |  |  |  |  |
Business Acquisition, Description of Acquired Entity |  |  |  | a provider of physician staffing services headquartered in Houston, Texas. |  |  |  |  |
Business Combination, Reason for Business Combination |  |  |  | The primary reason for the acquisition was to expand the Company's Physician staffing services. |  |  |  |  |
Business Combination, Consideration Transferred |  |  |  | 19,200,000us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment | 272,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
BusinessCombinationRecognizedGoodwill | 7,076,000asgn_BusinessCombinationRecognizedGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Goodwill, Acquired During Period |  |  | 7,076,000us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets | 24,821,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities | 5,093,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other | 551,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities | 5,644,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net | 19,177,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Acquisition, Cost of Acquired Entity, Cash Paid |  |  |  | 18,500,000us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |
contingent consideration maximum | 5,000,000asgn_Contingentconsiderationmaximum / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |  |  |
Business Combination, Acquisition Related Costs |  |  | 400,000us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember |  |  |  |  |  |
Apex Systems |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets |  |  |  |  | 494,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets |  |  |  |  | 172,042,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  | 251,555,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Acquisition, Effective Date of Acquisition |  |  |  |  | May 15, 2012 |  |  |  |
Business Acquisition, Name of Acquired Entity |  |  |  |  | Apex Systems, Inc. |  |  |  |
Business Acquisition, Description of Acquired Entity |  |  |  |  | a privately-owned provider of information technology staffing headquartered in Richmond, Virginia. |  |  |  |
Business Combination, Reason for Business Combination |  |  |  |  | The primary reason for the acquisition was to expand the Company's information technology staffing services. |  |  |  |
Business Combination, Consideration Transferred |  |  |  |  | 610,800,000us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment |  |  |  |  | 902,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
BusinessCombinationRecognizedGoodwill |  |  |  |  | 264,590,000asgn_BusinessCombinationRecognizedGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets |  |  |  |  | 689,583,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities |  |  |  |  | 77,905,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other |  |  |  |  | 850,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities |  |  |  |  | 78,755,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net |  |  |  |  | 610,828,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |  |
Business Acquisition, Cost of Acquired Entity, Cash Paid |  |  |  |  | 385,000,000us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember | 300,000us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |  |
Business Combination, Acquisition Related Costs |  |  |  |  |  |  | 9,800,000us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember |  |
Contractor Relations [Member] | CyberCoders [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  |  |  |  |  | 4,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = asgn_ContractorRelationsMember
Contractor Relations [Member] | Whitaker [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill | 1,100,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = asgn_ContractorRelationsMember |  |  |  |  |  |  |  |
Contractor Relations [Member] | Apex Systems |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  | 10,589,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = asgn_ContractorRelationsMember |  |  |  |
Customer Relationships [Member] | CyberCoders [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  |  |  |  |  | 860,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Customer Relationships [Member] | Whitaker [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill | 6,200,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |  |  |  |  |  |  |  |
Customer Relationships [Member] | Apex Systems |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  | 92,147,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |  |  |  |
Noncompete Agreements [Member] | CyberCoders [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  |  |  |  |  | 800,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember
Noncompete Agreements [Member] | Whitaker [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill | 50,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember |  |  |  |  |  |  |  |
Noncompete Agreements [Member] | Apex Systems |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  | 2,076,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember |  |  |  |
Software [Member] | CyberCoders [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  |  |  |  |  | 18,900,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_ComputerSoftwareIntangibleAssetMember
Trademarks [Member] | CyberCoders [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  |  |  |  |  | 13,300,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_CyberCodersMember / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember
Trademarks [Member] | Whitaker [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill | 400,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_WhitakerMember / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember |  |  |  |  |  |  |  |
Trademarks [Member] | Apex Systems |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill |  |  |  |  | $ 146,743,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = asgn_ApexSystemsMember / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember |  |  |  |

X
- Definition Business Acquisition Contingent Consideration At Fair Value Accrued Earn-Out + References No definition available. + Details
Name: | asgn_BusinessAcquisitionContingentConsiderationAtFairValueAccruedEarnOut
Namespace Prefix: | asgn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of goodwill recognized as of the acquisition date. + References No definition available. + Details
Name: | asgn_BusinessCombinationRecognizedGoodwill
Namespace Prefix: | asgn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents the maximum amount of consideration which may be paid. + References No definition available. + Details
Name: | asgn_Contingentconsiderationmaximum
Namespace Prefix: | asgn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition With respect to a business combination completed during the period, this element provides a description of the business, other than the name, which may include the industry, size, products and other important information. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e845-128460 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Acquisition Date -URI http://asc.fasb.org/extlink&oid=6919109 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e848-128460 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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

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- Definition Name of the acquired entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e1043-128460 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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