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
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Net earnings attributable to W.W. Grainger, Inc. as reported	$ 174,744	$ 186,713
Distributed earnings available to participating securities	(546)	(626)
Undistributed earnings available to participating securities	(932)	(1,121)
Numerator for basic earnings per share – Undistributed and distributed earnings available to common shareholders	173,266	184,966
Undistributed earnings allocated to participating securities	932	1,121
Undistributed earnings reallocated to participating securities	(924)	(1,114)
Numerator for diluted earnings per share – Undistributed and distributed earnings available to common shareholders	$ 173,274	$ 184,973
Denominator for basic earnings per share - weighted average shares (in shares)	58,720,066	61,668,682
Effect of dilutive securities (in shares)	482,816	431,119
Denominator for diluted earnings per share - weighted average shares adjusted for dilutive securities (in shares)	59,202,882	62,099,801
Basic (in dollars per share)	$ 2.95	$ 3.00
Diluted (in dollars per share)	$ 2.93	$ 2.98