| Document and Entity Information | Nov. 05, 2020 | Aug. 06, 2020 |
| Entity Information [Line Items] | ||
| Document Type | 8-K | |
| Document Period End Date | Nov. 5, 2020 | |
| Entity Registrant Name | KELLY SERVICES, INC. | |
| Entity Incorporation, State or Country Code | DE | |
| Entity File Number | 0-1088 | |
| Entity Tax Identification Number | 38-1510762 | |
| Entity Address, Address Line One | 999 West Big Beaver Road | |
| Entity Address, City or Town | Troy | |
| Entity Address, State or Province | MI | |
| Entity Address, Postal Zip Code | 48084 | |
| City Area Code | 248 | |
| Local Phone Number | 362-4444 | |
| Written Communications | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Entity Emerging Growth Company | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Entity Central Index Key | 0000055135 | |
| Amendment Flag | false | |
| NASDAQ Global Market | Class A Common | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Class A Common | |
| Trading Symbol | KELYA | |
| Security Exchange Name | NASDAQ | |
| NASDAQ Global Market | Class B Common | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Class B Common | |
| Trading Symbol | KELYB | |
| Security Exchange Name | NASDAQ |
