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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2019
Service fee revenue (approximately) 1.00%      
Accrued sales returns $ 29 $ 28    
Accrued sales incentives 62 55    
Advertising expense $ 241 187 $ 180  
Original maturity of cash (days) 90 days      
Percentage of LIFO Inventory 66.00%      
Depreciation $ 162 170 166  
Capitalized interest costs $ 10 $ 2 $ 2  
Minimum [Member]        
Buildings, structures and improvements, estimated useful life 10 years      
Furniture, fixtures, machinery and equipment, estimated useful life 3 years      
Capitalized software amortization period 3 years      
Finite-lived intangible assets, useful life 7 years      
Maximum [Member]        
Buildings, structures and improvements, estimated useful life 30 years      
Furniture, fixtures, machinery and equipment, estimated useful life 10 years      
Capitalized software amortization period 5 years      
Finite-lived intangible assets, useful life 22 years      
AOCE Attributable to W.W. Grainger, Inc.        
Reclassification due to the adoption of ASU 2018-02 $ 15      
Retained Earnings [Member]        
Reclassification due to the adoption of ASU 2018-02 $ (15)      
Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member]        
Operating lease, right-of-use asset, percentage of total assets       4.00%
Operating lease, liability, percentage of total liabilities       6.00%
Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member] | Minimum [Member]        
Operating Lease, Right-of-Use Asset       $ 210
Operating lease, liability       210
Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member] | Maximum [Member]        
Operating Lease, Right-of-Use Asset       230
Operating lease, liability       $ 230