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Income Taxes Additional Information (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Feb. 15, 2015
Feb. 16, 2014
Feb. 15, 2015
Feb. 16, 2014
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, Percent 30.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 32.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations 34.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Discrete Income Tax Benefit $ 45cost_DiscreteIncomeTaxBenefit   $ 44cost_DiscreteIncomeTaxBenefit  
Ownership of employee, shares 29us-gaap_SharesOutstanding   29us-gaap_SharesOutstanding  
Foreign Tax Authority [Member]        
Income Taxes [Line Items]        
Income Tax Expense (Benefit), Ongoing Tax Matter     14cost_IncomeTaxExpenseBenefitOngoingTaxMatter
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember		  
Special Dividend [Member]        
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, Deduction, Dividends, Amount     $ 57us-gaap_IncomeTaxReconciliationDeductionsDividends
/ cost_DividendsPaidAxis
= cost_SpecialDividendMember