| Cover | Oct. 27, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 27, 2020 |
| Entity File Number | 1-5684 |
| Entity Registrant Name | W.W. Grainger, Inc. |
| Entity Central Index Key | 0000277135 |
| Entity Tax Identification Number | 36-1150280 |
| Entity Incorporation, State or Country Code | IL |
| 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 |
| City Area Code | 847 |
| Local Phone Number | 535-1000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.50 per share |
| Trading Symbol | GWW |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
