Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Nov. 26, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed consolidated financial statements include the accounts of Costco, its wholly-owned subsidiaries, and subsidiaries in which it has a controlling interest. The Company reports noncontrolling interests in consolidated entities as a component of equity separate from the Company’s equity. All material inter-company transactions between and among the Company and its consolidated subsidiaries have been eliminated in consolidation. The Company’s net income excludes income attributable to noncontrolling interests in its operations in Taiwan. During the first quarter of 2018, Costco purchased its former joint venture partner's remaining equity interest in its Korean operations. Unless otherwise noted, references to net income relate to net income attributable to Costco. These unaudited condensed consolidated financial statements have been prepared in accordance with the instructions to Form 10-Q for interim financial reporting pursuant to the rules and regulations of the Securities and Exchange Commission (SEC). While these statements reflect all normal recurring adjustments that are, in the opinion of management, necessary for fair presentation of the results of the interim period, they do not include all of the information and footnotes required by U.S. generally accepted accounting principles (U.S. GAAP) for complete financial statements. Therefore, the interim condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes included in the Company's Annual Report filed on Form 10-K for the fiscal year ended September 3, 2017.
Fiscal Year End	Fiscal Year End The Company operates on a 52/53 week fiscal year basis, with the fiscal year ending on the Sunday closest to August 31. Fiscal 2018 is a 52-week year ending on September 2, 2018. References to the first quarters of 2018 and 2017 relate to the 12-week fiscal quarters ended November 26, 2017, and November 20, 2016, respectively.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results and events could differ from those estimates and assumptions.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company accounts for certain assets and liabilities at fair value. The carrying value of the Company’s financial instruments, including cash and cash equivalents, receivables and accounts payable, approximate fair value due to their short-term nature or variable interest rates. See Notes 2, 3, and 4 for the carrying value and fair value of the Company’s investments, derivative instruments, and fixed-rate debt, respectively. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value is estimated by applying a fair value hierarchy, which requires maximizing the use of observable inputs when measuring fair value. The three levels of inputs are: Level 1: Quoted market prices in active markets for identical assets or liabilities. Level 2: Observable market-based inputs or unobservable inputs that are corroborated by market data. Level 3: Significant unobservable inputs that are not corroborated by market data. Current financial liabilities have fair values that approximate their carrying values. Long-term financial liabilities include the Company's long-term debt, which are recorded on the balance sheet at issuance price and adjusted for unamortized discounts or premiums and debt issuance costs, which are being amortized to interest expense over the term of the loan. There have been no material changes to the valuation techniques utilized in the fair value measurement of assets and liabilities as disclosed in the Company's 2017 Form 10-K.
Merchandise Inventories	Merchandise Inventories Merchandise inventories are stated at the lower of cost or market. U.S. merchandise inventories are valued by the cost method of accounting, using the last-in, first-out (LIFO) basis. The Company believes the LIFO method more fairly presents the results of operations by more closely matching current costs with current revenues. The Company records adjustments quarterly, if necessary, for the projected annual effect of inflation or deflation, and these estimates are adjusted to actual results determined at year-end, after actual inflation rates and inventory levels have been determined. Canadian and Other International merchandise inventories are predominantly valued using the cost and retail inventory methods, respectively, using the first-in, first-out (FIFO) basis. As of November 26, 2017, U.S. merchandise inventories valued at LIFO approximated FIFO after considering the lower of cost or market principle. As of September 3, 2017, the cumulative impact of the LIFO valuation on merchandise inventories was zero.
Derivatives	Derivatives The Company is exposed to foreign-currency exchange-rate fluctuations in the normal course of business. It manages these fluctuations, in part, through the use of forward foreign-exchange contracts, seeking to economically hedge the impact of fluctuations of foreign exchange on known future expenditures denominated in a non-functional foreign-currency. The contracts relate primarily to U.S. dollar merchandise inventory expenditures made by the Company’s international subsidiaries, whose functional currency is not the U.S. dollar. These contracts do not qualify for derivative hedge accounting. The Company seeks to mitigate risk with the use of these contracts and does not intend to engage in speculative transactions. Some of these contracts contain credit-risk-related contingent features that require settlement of outstanding contracts upon certain triggering events. At November 26, 2017 and September 3, 2017, both the aggregate fair value amounts of derivative instruments in a net liability position and the amount needed to settle the instruments immediately if the credit-risk-related contingent features were triggered were immaterial. The aggregate notional amounts of unsettled forward foreign-exchange contracts were $928 and $637 at November 26, 2017, and September 3, 2017, respectively. The Company seeks to manage counterparty risk associated with these contracts by limiting transactions to counterparties with which the Company has an established banking relationship. There can be no assurance that this practice is effective. The contracts are limited to less than one year in duration. See Note 3 for information on the fair value of unsettled forward foreign-exchange contracts as of November 26, 2017, and September 3, 2017. The unrealized gains or losses recognized in interest income and other, net in the accompanying condensed consolidated statements of income relating to the net changes in the fair value of unsettled forward foreign-exchange contracts were immaterial in the first quarter of 2018 and resulted in a net gain of $30 in the first quarter of 2017. The Company is exposed to fluctuations in prices for the energy it consumes, particularly electricity and natural gas, which it seeks to partially mitigate through the use of fixed-price contracts for certain of its warehouses and other facilities, primarily in the U.S. and Canada. The Company also enters into variable-priced contracts for some purchases of natural gas and fuel for its gas stations on an index basis. These contracts meet the characteristics of derivative instruments, but generally qualify for the “normal purchases or normal sales” exception under authoritative guidance and require no mark-to-market adjustment.
Foreign Currency	Foreign Currency The Company recognizes foreign-currency transaction gains and losses related to revaluing or settling monetary assets and liabilities denominated in currencies other than the functional currency in interest income and other, net in the accompanying condensed consolidated statements of income. Generally, these include the U.S. dollar cash and cash equivalents and the U.S. dollar payables of consolidated subsidiaries revalued to their functional currency. Also included are realized foreign-currency gains or losses from settlements of forward foreign-exchange contracts. These items were immaterial in the first quarter of 2018 and 2017.
Stock Repurchase Programs	Stock Repurchase Programs Repurchased shares of common stock are retired, in accordance with the Washington Business Corporation Act. The par value of repurchased shares is deducted from common stock and the excess repurchase price over par value is deducted by allocation to additional paid-in capital and retained earnings. The amount allocated to additional paid-in capital is the current value of additional paid-in capital per share outstanding and is applied to the number of shares repurchased. Any remaining amount is allocated to retained earnings. See Note 5 for additional information.
Recent Accounting Pronouncements Adopted	Recent Accounting Pronouncements Adopted In March 2016, the Financial Accounting Standards Board (FASB) issued guidance intended to simplify accounting for share-based payment transactions. The guidance relates to the accounting for income taxes, forfeitures, and minimum statutory tax withholding requirements. The new standard was effective for fiscal years and interim periods within those years beginning after December 15, 2016, with early adoption permitted. The Company adopted this guidance at the beginning of its first quarter of fiscal year 2018 and recognized an excess tax benefit of $41 as part of its income tax provision in the accompanying condensed consolidated statements of income. Previously these amounts were reflected in equity. Additionally, these amounts are now reflected as cash flows from operations instead of cash flows from financing activities in the consolidated statements of cash flows on a prospective basis. Adoption of this guidance did not have a material impact on the consolidated balance sheets, consolidated statements of cash flows, or related disclosures.
Recent Accounting Pronouncements Not Yet Adopted	Recent Accounting Pronouncements Not Yet Adopted In May 2014, the FASB issued guidance on the recognition of revenue from contracts with customers. The guidance converges the requirements for reporting revenue and requires disclosures sufficient to describe the nature, amount, timing, and uncertainty of revenue and cash flows arising from these contracts. Transition is permitted either retrospectively or as a cumulative effect adjustment as of the date of adoption. The new standard is effective for fiscal years and interim periods within those years beginning after December 15, 2017. The Company plans to adopt this guidance at the beginning of its first quarter of fiscal year 2019. While its review is continuing, based on preliminary assessments, the Company believes the new guidance will change recognition timing of cash-card breakage income to reflect the historical pattern of gift card redemption rather than the current methodology of recognizing income when redemption is considered remote. The Company will also present estimated sales returns on a gross basis rather than net of the sales return reserve on the consolidated balance sheets. The Company continues to evaluate various areas such as gross versus net revenue presentation for certain contracts, identification and treatment of performance obligations associated with membership offers, and accounting for warranty arrangements on qualified purchases. The Company continues to evaluate potential impacts on the contracts associated with the co-branded credit card arrangement as well as its adoption methodology. In February 2016, the FASB issued guidance on leases, which will require recognition on the balance sheet for the rights and obligations created by all leases with terms greater than twelve months. The standard is effective for fiscal years and interim periods within those years beginning after December 15, 2018, with early adoption permitted. The Company plans to adopt this guidance at the beginning of its first quarter of fiscal year 2020. While the Company continues to evaluate this standard and the effect on related disclosures, the primary effect of adoption will be to require recording right-of-use assets and corresponding lease obligations for current operating leases. The adoption is expected to have a material impact on the Company's consolidated balance sheets, but not on the consolidated statements of income or consolidated statements of cash flows.