COVER	6 Months Ended
	Jun. 30, 2020 shares
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
Document Transition Report	false
Entity File Number	1-5684
Entity Registrant Name	W.W. Grainger, Inc.
Entity Incorporation, State or Country Code	IL
Entity Tax Identification Number	36-1150280
Entity Address, Address Line One	100 Grainger Parkway,
Entity Address, City or Town	Lake Forest,
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60045-5201
City Area Code	847
Local Phone Number	535-1000
Title of 12(b) Security	Common Stock
Trading Symbol	GWW
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	53,571,736
Entity Central Index Key	0000277135
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Amendment Flag	false