| Document and Entity Information | Jul. 29, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jul. 29, 2020 |
| Entity Registrant Name | ASGN Inc |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-35636 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | ASGN |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000890564 |
| Amendment Flag | false |
