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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net earnings attributable to W.W. Grainger, Inc. as reported	$ 97,921	$ 172,676	$ 272,665	$ 359,389
Distributed earnings available to participating securities	(427)	(576)	(973)	(1,202)
Undistributed earnings available to participating securities	(217)	(970)	(1,159)	(2,092)
Numerator for basic earnings per share – Undistributed and distributed earnings available to common shareholders	97,277	171,130	270,533	356,095
Undistributed earnings allocated to participating securities	217	970	1,159	2,092
Undistributed earnings reallocated to participating securities	(216)	(964)	(1,152)	(2,078)
Numerator for diluted earnings per share – Undistributed and distributed earnings available to common shareholders	$ 97,278	$ 171,136	$ 270,540	$ 356,109
Denominator for basic earnings per share - weighted average shares (in shares)	58,012,731	60,891,298	58,363,416	61,278,981
Effect of dilutive securities (in shares)	274,581	410,247	377,846	420,622
Denominator for diluted earnings per share - weighted average shares adjusted for dilutive securities (in shares)	58,287,312	61,301,545	58,741,262	61,699,603
Basic (in dollars per share)	$ 1.68	$ 2.81	$ 4.64	$ 5.81
Diluted (in dollars per share)	$ 1.67	$ 2.79	$ 4.61	$ 5.77