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Debt (Additional Information) (Detail) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Feb. 15, 2015
Feb. 17, 2015
1.75% Senior Notes due February 2020    
Debt Instrument [Line Items]    
Debt instrument, maturity date Feb. 15, 2020  
Debt instrument, frequency of periodic payment Interest is due semi-annually on February 15 and August 15, with the first payment due on August 15, 2015  
2.25% Senior Notes due February 2022    
Debt Instrument [Line Items]    
Debt instrument, maturity date Feb. 15, 2022  
Debt instrument, frequency of periodic payment Interest is due semi-annually on February 15 and August 15, with the first payment due on August 15, 2015  
Subsequent Event [Member]    
Debt Instrument [Line Items]    
Senior note   $ 1,000us-gaap_SeniorNotes
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Subsequent Event [Member] | Senior Notes [Member]    
Debt Instrument [Line Items]    
Redemption price company option   100.00%cost_RedemptionPriceCompanyOption
/ us-gaap_DebtInstrumentAxis
= us-gaap_SeniorNotesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Redemption price certain events   101.00%cost_RedemptionPriceCertainEvents
/ us-gaap_DebtInstrumentAxis
= us-gaap_SeniorNotesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Subsequent Event [Member] | 1.75% Senior Notes due February 2020    
Debt Instrument [Line Items]    
Senior note   500us-gaap_SeniorNotes
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesOnePointSevenFivePercentDueFebruaryTwentyTwentyMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Senior note, interest rate   1.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesOnePointSevenFivePercentDueFebruaryTwentyTwentyMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Subsequent Event [Member] | 2.25% Senior Notes due February 2022    
Debt Instrument [Line Items]    
Senior note   $ 500us-gaap_SeniorNotes
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesTwoPointTwoFivePercentDueFebruaryTwentyTwentyTwoMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Senior note, interest rate   2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesTwoPointTwoFivePercentDueFebruaryTwentyTwentyTwoMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember