Document and Entity Information - USD ($) shares in Millions, $ in Billions	12 Months Ended
	Dec. 31, 2023	Feb. 15, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
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	4400 Cox Road, Suite 110
Entity Address, City or Town	Glen Allen,
Entity Address, State or Province	VA
Entity Address, Postal Zip Code	23060
City Area Code	888
Local Phone Number	482-8068
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 3.6
Entity Common Stock, Shares Outstanding (in shares)		46.5
Documents Incorporated by Reference	We are incorporating by reference into Parts II and III of this Annual Report on Form 10-K portions of the registrant’s definitive proxy statement for the 2024 Annual Meeting of Stockholders, to be filed within 120 days of the close of the registrant’s fiscal year 2023.
Entity Central Index Key	0000890564
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Period End Date	Dec. 31, 2023
Document Financial Statement Error Correction [Flag]	false