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SEGMENT INFORMATION - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) segment
Segment Reporting Information [Line Items]
Number of reportable segments | segment		2
Service revenues, percent of total revenues		1.00%
UNITED STATES [Member]
Segment Reporting Information [Line Items]
Intersegment sales increase | $	$ 12	$ 40

X

- Definition

Increase in Intersegment Sales for the respective Reporting Segment.

+ References

No definition available.

+ Details

Name:	gww_IntersegmentSalesIncrease
Namespace Prefix:	gww_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Grainger's Service Fee Revenue as a percentage of sales.

+ References

No definition available.

+ Details

Name:	gww_ServiceFeeRevenue
Namespace Prefix:	gww_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: