EARNINGS PER SHARE - Schedule of Computation of Basic and Diluted Earnings per Share under Two-Class Method (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net earnings attributable to W.W. Grainger, Inc. as reported	$ 253	$ 232
Distributed earnings available to participating securities	(1)	(1)
Undistributed earnings available to participating securities	(2)	(1)
Numerator for basic earnings per share – Undistributed and distributed earnings available to common shareholders	250	230
Undistributed earnings allocated to participating securities	2	1
Undistributed earnings reallocated to participating securities	(2)	(1)
Numerator for diluted earnings per share – Undistributed and distributed earnings available to common shareholders	$ 250	$ 230
Denominator for basic earnings per share - weighted average shares (in shares)	55,643,199	56,062,607
Effect of dilutive securities (in shares)	304,429	340,639
Denominator for diluted earnings per share - weighted average shares adjusted for dilutive securities (in shares)	55,947,628	56,403,246
Basic (in dollars per share)	$ 4.50	$ 4.09
Diluted (in dollars per share)	$ 4.48	$ 4.07

Dividends paid to restricted stock unit holders.

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Net earnings re-allocated to participating securities after dilutive impact of stock options.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Number of additional shares included in diluted EPS for potentially dilutive effect of nonvested equity-based payment award containing forfeitable rights to dividends or dividend equivalents, whether paid or unpaid.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities, and addition from assumption of issuance of common shares for dilutive potential common shares; of income (loss) available to common shareholders.

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Amount of undistributed earnings (loss) allocated to participating securities for the diluted earnings (loss) per share or per unit calculation under the two-class method.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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