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EARNINGS PER SHARE (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Share [Abstract]                      
Net earnings attributable to W.W. Grainger, Inc. as reported $ 209 $ 104 $ 237 $ 232 $ 151 $ 162 $ 98 $ 175 $ 782 $ 586 $ 606
Distributed earnings available to participating securities, basic                 (2) (2) (2)
Undistributed earnings available to participating securities, basic                 (4) (3) (3)
Numerator for basic earnings per share - Undistributed and distributed earnings available to common shareholders                 776 581 601
Undistributed earnings allocated to participating securities, diluted                 4 3 3
Undistributed earnings reallocated to participating securities, diluted                 (4) (3) (3)
Numerator for diluted earnings per share - Undistributed and distributed earnings available to common shareholders                 $ 776 $ 581 $ 601
Denominator for basic earnings per share – weighted average shares (in shares)                 56,142,604 57,674,977 60,430,892
Effect of dilutive securities (in shares)                 391,581 308,190 409,038
Denominator for diluted earnings per share – weighted average shares adjusted for dilutive securities (in shares)                 56,534,185 57,983,167 60,839,930
Basic (in dollars per share) $ 3.71 $ 1.84 $ 4.19 $ 4.09 $ 2.64 $ 2.80 $ 1.68 $ 2.95 $ 13.82 $ 10.07 $ 9.94
Diluted (in dollars per share) $ 3.68 $ 1.82 $ 4.16 $ 4.07 $ 2.63 $ 2.79 $ 1.67 $ 2.93 $ 13.73 $ 10.02 $ 9.87