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EARNINGS PER SHARE - Schedule of Computation of Basic and Diluted Earnings per Share under Two-Class Method (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Earnings Per Share [Abstract]        
Net earnings attributable to W.W. Grainger, Inc. as reported $ 104,377 $ 162,006 $ 572,893 $ 434,671
Distributed earnings available to participating securities (640) (603) (1,596) (1,576)
Undistributed earnings available to participating securities (243) (806) (3,108) (1,966)
Numerator for basic earnings per share – Undistributed and distributed earnings available to common shareholders 103,494 160,597 568,189 431,129
Undistributed earnings allocated to participating securities 243 806 3,108 1,966
Undistributed earnings reallocated to participating securities (242) (803) (3,086) (1,956)
Numerator for diluted earnings per share – Undistributed and distributed earnings available to common shareholders $ 103,495 $ 160,600 $ 568,211 $ 431,139
Denominator for basic earnings per share - weighted average shares (in shares) 56,339,630 57,316,532 56,172,277 58,010,222
Effect of dilutive securities (in shares) 464,227 204,816 416,253 319,703
Denominator for diluted earnings per share - weighted average shares adjusted for dilutive securities (in shares) 56,803,857 57,521,348 56,588,530 58,329,925
Basic (in dollars per share) $ 1.84 $ 2.80 $ 10.12 $ 7.43
Diluted (in dollars per share) $ 1.82 $ 2.79 $ 10.04 $ 7.39