GOODWILL AND OTHER INTANGIBLE ASSETS (Policies)	6 Months Ended
Jun. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets, Goodwill, Policy	Grainger tests reporting units' goodwill and intangible assets for impairment annually during the fourth quarter and more frequently if impairment indicators exist. Accordingly, Grainger periodically performs qualitative assessments of significant events and circumstances such as reporting units' historical and current results, assumptions regarding future performance, strategic initiatives and overall economic factors to determine the existence of impairment indicators and assess if it is more likely than not that the fair value of reporting units or intangible assets is less than their carrying value and if a quantitative impairment test is necessary.

Disclosure of accounting policy for goodwill. This accounting policy also may address how an entity assesses and measures impairment of goodwill, how reporting units are determined, how goodwill is allocated to such units, and how the fair values of the reporting units are determined.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84158767&loc=d3e18780-107790

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