SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) [Abstract]
Net sales	$ 2,763	$ 2,831	$ 2,861	$ 2,766	$ 2,633	$ 2,636	$ 2,615	$ 2,541	$ 11,221	$ 10,425	$ 10,137
Cost of goods sold	1,697	1,752	1,750	1,674	1,611	1,619	1,575	1,522	6,873	6,327	6,022
Gross profit	1,066	1,079	1,111	1,092	1,022	1,017	1,040	1,019	4,348	4,098	4,115
Selling, general and administrative expenses	776	890	767	757	786	740	811	726	3,190	3,063	3,002
Operating earnings	290	189	344	335	236	277	229	293	1,158	1,035	1,113
Net earnings attributable to W.W. Grainger, Inc.	$ 209	$ 104	$ 237	$ 232	$ 151	$ 162	$ 98	$ 175	$ 782	$ 586	$ 606
Earnings per share - basic (in dollars per share)	$ 3.71	$ 1.84	$ 4.19	$ 4.09	$ 2.64	$ 2.80	$ 1.68	$ 2.95	$ 13.82	$ 10.07	$ 9.94
Earnings per share - diluted (in dollars per share)	$ 3.68	$ 1.82	$ 4.16	$ 4.07	$ 2.63	$ 2.79	$ 1.67	$ 2.93	$ 13.73	$ 10.02	$ 9.87

SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) [Abstract]

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The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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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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Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

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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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Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

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The aggregate total costs related to selling a firm's product and services, as well as all other general and administrative expenses. Direct selling expenses (for example, credit, warranty, and advertising) are expenses that can be directly linked to the sale of specific products. Indirect selling expenses are expenses that cannot be directly linked to the sale of specific products, for example telephone expenses, Internet, and postal charges. General and administrative expenses include salaries of non-sales personnel, rent, utilities, communication, etc.

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