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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, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Earnings Per Share [Abstract]        
Net earnings attributable to W.W. Grainger, Inc. as reported $ 162,006 $ 185,873 $ 434,671 $ 545,262
Distributed earnings available to participating securities (603) (547) (1,576) (1,749)
Undistributed earnings available to participating securities (806) (1,085) (1,966) (3,179)
Numerator for basic earnings per share – Undistributed and distributed earnings available to common shareholders 160,597 184,241 431,129 540,334
Undistributed earnings allocated to participating securities 806 1,085 1,966 3,179
Undistributed earnings reallocated to participating securities (803) (1,078) (1,956) (3,157)
Numerator for diluted earnings per share – Undistributed and distributed earnings available to common shareholders $ 160,600 $ 184,248 $ 431,139 $ 540,356
Denominator for basic earnings per share - weighted average shares (in shares) 57,316,532 60,016,550 58,010,222 60,854,548
Effect of dilutive securities (in shares) 204,816 399,601 319,703 413,571
Denominator for diluted earnings per share - weighted average shares adjusted for dilutive securities (in shares) 57,521,348 60,416,151 58,329,925 61,268,119
Basic (in dollars per share) $ 2.80 $ 3.07 $ 7.43 $ 8.88
Diluted (in dollars per share) $ 2.79 $ 3.05 $ 7.39 $ 8.82