Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Feb. 16, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed consolidated financial statements include the accounts of Costco, its wholly-owned subsidiaries, subsidiaries in which it has a controlling interest, consolidated entities in which it has made equity investments, or has other interests through which it has majority-voting control or it exercises the right to direct the activities that most significantly impact the entity’s performance. 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 and other consolidated entities have been eliminated in consolidation. The Company’s net income excludes income attributable to noncontrolling interests 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 September 1, 2013.
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. References to the second quarters of 2014 and 2013 relate to the twelve-week fiscal quarters ended February 16, 2014, and February 17, 2013, 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.
Reclassifications	Reclassifications Certain reclassifications have been made to prior fiscal year amounts or balances to conform to the presentation in the current fiscal year. These reclassifications did not have a material impact on the Company’s previously reported condensed consolidated financial statements.
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, approximates fair value due to the instruments' 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 (an exit price) 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. 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 Form 10-K for the fiscal year ended September 1, 2013.
Merchandise Inventories	Merchandise Inventories Merchandise inventories are valued at the lower of cost or market, as determined primarily by the retail inventory method, and are stated using the last-in, first-out (LIFO) method for substantially all U.S. merchandise inventories. Merchandise inventories for all foreign operations are primarily valued by the retail inventory method and are stated using the first-in, first-out (FIFO) method. 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, when actual inflation rates and inventory levels for the year have been determined. Due to overall net inflationary trends in the second quarter and first half of 2014, a charge of $5 and $6 was recorded to merchandise costs, respectively, to increase the cumulative LIFO valuation on merchandise inventories. Due to overall net deflationary trends, a benefit of $9 and $11 was recorded to merchandise costs in the second quarter and first half of 2013, respectively. At February 16, 2014, and September 1, 2013, the cumulative impact of the LIFO valuation on merchandise inventories was $87 and $81, respectively.
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 open, unsettled forward foreign-exchange contracts were $517 and $458 at February 16, 2014, and September 1, 2013, respectively. While 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 February 16, 2014, and September 1, 2013. 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 second quarter and the first half of 2014 and 2013, 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 electricity and natural gas, in addition to 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 thus require no mark-to-market adjustment.
Foreign Currency	Foreign Currency The Company recognizes foreign-currency transaction gains and losses related to revaluing or settling all 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, this includes the U.S. dollar cash and cash equivalents and the U.S. dollar payables of consolidated subsidiaries to their functional currency. Also included are realized foreign-currency gains or losses from settlements of forward foreign-exchange contracts. These items resulted in a net gain of $10 and $19 in the second quarter and first half of 2014, respectively, as compared to a net gain of $6 and $15 in the second quarter and first half of 2013, 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 both additional paid-in capital and retained earnings. The amount allocated to additional paid-in capital is calculated as 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.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In February 2013, the Financial Accounting Standards Board (FASB) issued guidance related to reclassifications out of accumulated other comprehensive income. Disclosure will be required of the net income line items impacted by significant reclassifications out of accumulated other comprehensive income if the item is reclassified in its entirety. For other amounts that are not required to be reclassified in their entirety to net income, cross-references to other disclosures required under U.S. GAAP are required. The new guidance was effective for fiscal years, and interim periods within those years, beginning after December 15, 2012. The Company adopted this guidance at the beginning of its first quarter of 2014. Adoption did not have a material impact on the Company’s condensed consolidated financial statements or disclosures.