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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jan. 01, 2019
Service fee revenue (approximately) | 1.00% |  |  |
Accrued sales returns | $ 29 | $ 28 |  |
Accrued sales incentives | 62 | 55 |  |
Advertising expense | $ 241 | 187 | $ 180 |
Original maturity of cash (days) | 90 days |  |  |
Percentage of LIFO Inventory | 66.00% |  |  |
Depreciation | $ 162 | 170 | 166 |
Capitalized interest costs | $ 10 | $ 2 | $ 2 |
Minimum [Member] |  |  |  |
Buildings, structures and improvements, estimated useful life | 10 years |  |  |
Furniture, fixtures, machinery and equipment, estimated useful life | 3 years |  |  |
Capitalized software amortization period | 3 years |  |  |
Finite-lived intangible assets, useful life | 7 years |  |  |
Maximum [Member] |  |  |  |
Buildings, structures and improvements, estimated useful life | 30 years |  |  |
Furniture, fixtures, machinery and equipment, estimated useful life | 10 years |  |  |
Capitalized software amortization period | 5 years |  |  |
Finite-lived intangible assets, useful life | 22 years |  |  |
AOCE Attributable to W.W. Grainger, Inc. |  |  |  |
Reclassification due to the adoption of ASU 2018-02 | $ 15 |  |  |
Retained Earnings [Member] |  |  |  |
Reclassification due to the adoption of ASU 2018-02 | $ (15) |  |  |
Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member] |  |  |  |
Operating lease, right-of-use asset, percentage of total assets |  |  |  | 4.00%
Operating lease, liability, percentage of total liabilities |  |  |  | 6.00%
Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member] | Minimum [Member] |  |  |  |
Operating Lease, Right-of-Use Asset |  |  |  | $ 210
Operating lease, liability |  |  |  | 210
Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member] | Maximum [Member] |  |  |  |
Operating Lease, Right-of-Use Asset |  |  |  | 230
Operating lease, liability |  |  |  | $ 230

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