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EARNINGS PER SHARE (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Abstract]                      
Net earnings attributable to W.W. Grainger, Inc. as reported $ 151,059 $ 162,006 $ 97,921 $ 174,744 $ 60,666 $ 185,873 $ 172,676 $ 186,713 $ 585,730 $ 605,928 $ 768,996
Distributed earnings available to participating securities, basic                 (2,005) (2,383) (2,823)
Undistributed earnings available to participating securities, basic                 (2,678) (3,044) (4,735)
Numerator for basic earnings per share - Undistributed and distributed earnings available to common shareholders                 581,047 600,501 761,438
Undistributed earnings allocated to participating securities, diluted                 2,678 3,044 4,735
Undistributed earnings reallocated to participating securities, diluted                 (2,663) (3,023) (4,692)
Numerator for diluted earnings per share - Undistributed and distributed earnings available to common shareholders                 $ 581,062 $ 600,522 $ 761,481
Denominator for basic earnings per share – weighted average shares (in shares)                 57,674,977 60,430,892 65,156,864
Effect of dilutive securities (in shares)                 308,190 409,038 608,257
Denominator for diluted earnings per share – weighted average shares adjusted for dilutive securities (in shares)                 57,983,167 60,839,930 65,765,121
Basic (in dollars per share) $ 2.64 $ 2.80 $ 1.68 $ 2.95 $ 1.02 $ 3.07 $ 2.81 $ 3.00 $ 10.07 $ 9.94 $ 11.69
Diluted (in dollars per share) $ 2.63 $ 2.79 $ 1.67 $ 2.93 $ 1.01 $ 3.05 $ 2.79 $ 2.98 $ 10.02 $ 9.87 $ 11.58