Document and Entity Information - shares shares in Millions	3 Months Ended
	Mar. 31, 2020	May 04, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
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 Description	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 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 Common Stock, Shares Outstanding (in shares)		52.5
Entity Central Index Key	0000890564
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false