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DOCUMENT AND ENTITY INFORMATION - USD ($)
 12 Months Ended
Dec. 31, 2018
Jan. 31, 2019
Jun. 30, 2018
Document and Entity Information [Abstract]      
Entity Registrant Name GRAINGER W W INC    
Entity Central Index Key 0000277135    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   55,679,223  
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 16,101,319,439