Document and Entity Information - shares	9 Months Ended
	Oct. 03, 2021	Nov. 01, 2021
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Oct. 03, 2021
Document Transition Report	false
Entity File Number	0-1088
Entity Registrant Name	KELLY SERVICES, INC.
Entity Incorporation, State or Country Code	DE
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Current Fiscal Year End Date	--01-02
Amendment Flag	false
Entity Central Index Key	0000055135
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Class A Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		36,030,919
Class B Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,358,521
NASDAQ Global Market | Class A Common Stock
Document Information [Line Items]
Title of 12(b) Security	Class A Common
Trading Symbol	KELYA
Security Exchange Name	NASDAQ
NASDAQ Global Market | Class B Common Stock
Document Information [Line Items]
Title of 12(b) Security	Class B Common
Trading Symbol	KELYB
Security Exchange Name	NASDAQ