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Document and Entity Information - USD ($)
shares in Millions, $ in Billions
 12 Months Ended
Dec. 31, 2019
Feb. 20, 2020
Jun. 28, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 001-35636    
Entity Registrant Name ASGN Inc    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 95-4023433    
Entity Address, Address Line One 26745 Malibu Hills Road    
Entity Address, City or Town Calabasas    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 91301    
City Area Code 818    
Local Phone Number 878-7900    
Title of 12(b) Security Common Stock    
Trading Symbol ASGN    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 3.1
Entity Common Stock, Shares Outstanding (in shares)   53.0  
Documents Incorporated by Reference
We are incorporating by reference into Part III of this Annual Report on Form 10-K portions of the registrant’s definitive proxy statement for the 2020 Annual Meeting of Stockholders, to be filed within 120 days of the close of the registrant’s fiscal year 2019.
    
Entity Central Index Key 0000890564    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false