Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Jan. 03, 2021	Feb. 08, 2021	Jun. 28, 2020
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Jan. 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 Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 524.3
Documents Incorporated by Reference	The proxy statement of the registrant with respect to its 2021 Annual Meeting of Stockholders is incorporated by reference in Part III.
Current Fiscal Year End Date	--01-03
Amendment Flag	false
Entity Central Index Key	0000055135
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Class A common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		35,929,080
Class B common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,372,521
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