Summary of Significant Accounting Policies (Policies)	8 Months Ended
Summary of Significant Accounting Policies (Policies)	May 07, 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 and Korea. 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 August 28, 2016.
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 2017 is a 53-week year ending on September 3, 2017. References to the third quarters of 2017 and 2016 relate to the 12-week fiscal quarters ended May 7, 2017, and May 8, 2016, respectively. References to the first thirty-six weeks of 2017 and 2016 relate to the thirty-six weeks ended May 7, 2017, and May 8, 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 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. The Company’s current financial liabilities have fair values that approximate their carrying values. The Company’s long-term financial liabilities consist of long-term debt, which is recorded on the balance sheet at issuance price and adjusted for any applicable unamortized discounts or premiums and debt issuance costs. 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 2016 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 an adjustment each quarter, 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 for the year 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 May 7, 2017, U.S. merchandise inventories valued at LIFO approximated FIFO after considering the lower of cost or market principle. As of August 28, 2016, the cumulative impact of the LIFO valuation on merchandise inventories was immaterial.
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. Currently, 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. These contracts do not contain any credit-risk-related contingent features. The aggregate notional amounts of unsettled forward foreign-exchange contracts were $554 and $572 at May 7, 2017, and August 28, 2016, 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 May 7, 2017, and August 28, 2016. 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 third quarter and first thirty-six weeks of 2017. Unrealized gains or losses on unsettled forward foreign-exchange contracts were a net loss of $11 and $17 in the third quarter and first thirty-six weeks of 2016, respectively. 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. The net changes related to these items were immaterial in the third quarter and first thirty-six weeks of 2017, respectively, as compared to an immaterial net gain and a $34 net gain in the third quarter and first thirty-six weeks of 2016, respectively.
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 Not Yet Adopted	Recent Accounting Pronouncements Not Yet Adopted In May 2014, the Financial Accounting Standards Board (FASB) issued new 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 and is reviewing current accounting policies, business processes, systems and controls to identify potential differences or changes that would result from applying the new standard. The Company is still evaluating whether or not there will be a material impact on the Company's consolidated financial statements and related disclosures as a result of adopting this standard. In February 2016, the FASB issued new guidance on leases, which will require lessees to recognize assets and liabilities 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 results of operations or cash flows. In March 2016, the FASB issued new guidance on stock compensation, intended to simplify accounting for share-based payment transactions. The guidance will change accounting for income taxes, forfeitures, and minimum statutory tax withholding requirements. The new standard is effective for fiscal years and interim periods within those years beginning after December 15, 2016, with early adoption permitted. The Company plans to adopt this guidance at the beginning of its first quarter of fiscal year 2018. Adoption of this guidance will likely be material to the provision for income taxes and earnings per share amounts on the Company’s consolidated income statements for the change in the recognition of excess tax benefits or deficiencies. Due to the Company's annual vesting and release of shares in its first fiscal quarter, this may create increased volatility in these amounts during that quarter of each fiscal year. Previously these amounts were reflected in equity. Additionally, these amounts will be reflected as an operating activity instead of financing activity in the consolidated statements of cash flows. Adoption of this guidance is not expected to have a material effect on the consolidated balance sheets, statements of cash flows, or related disclosures.