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COVER - USD ($)
 12 Months Ended
Dec. 31, 2020
Jan. 31, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 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 Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag true    
Entity Public Float     $ 15,084,028,289
Entity Common Stock, Shares Outstanding   52,375,717  
Documents Incorporated by Reference Portions of the registrant's definitive proxy statement to be filed in connection with the annual meeting of shareholders to be held on April 28, 2021, are incorporated by reference into Part III hereof of this Form 10-K where indicated. The registrant's definitive 2020 proxy statement will be filed on or about March 18, 2021.    
Entity Central Index Key 0000277135    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Amendment Flag false