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Debt (Additional Information) (Detail) (USD $)
In Millions, unless otherwise specified
9 Months Ended 1 Months Ended
May 10, 2015
May 31, 2015
Feb. 17, 2015
Aug. 31, 2014
May 21, 2015
Debt Instrument [Line Items]          
Senior note     $ 1,000us-gaap_SeniorNotes    
Long-term debt 6,026us-gaap_LongTermDebt     5,093us-gaap_LongTermDebt  
Senior Notes [Member]          
Debt Instrument [Line Items]          
Redemption price company option     100.00%cost_RedemptionPriceCompanyOption
/ us-gaap_DebtInstrumentAxis
= us-gaap_SeniorNotesMember
   
Redemption price certain events     101.00%cost_RedemptionPriceCertainEvents
/ us-gaap_DebtInstrumentAxis
= us-gaap_SeniorNotesMember
   
1.75% Senior Notes due February 2020          
Debt Instrument [Line Items]          
Senior note     500us-gaap_SeniorNotes
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesOnePointSevenFivePercentDueFebruaryTwentyTwentyMember
   
Senior note, interest rate     1.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesOnePointSevenFivePercentDueFebruaryTwentyTwentyMember
   
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]          
Senior note     500us-gaap_SeniorNotes
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesTwoPointTwoFivePercentDueFebruaryTwentyTwentyTwoMember
   
Senior note, interest rate     2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cost_SeniorNotesTwoPointTwoFivePercentDueFebruaryTwentyTwentyTwoMember
   
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] | 0.79% Promissory Notes due May 2025          
Debt Instrument [Line Items]          
Debt instrument, maturity date   May 31, 2025      
Debt instrument, frequency of periodic payment   Interest is payable semi-annually      
Long-term debt         $ 125us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= cost_PrivatePlacementNotesZeroPointSevenNinePercentDueMayTwentyTwentyFiveDomain
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Long-term debt, percentage bearing fixed interest, percentage rate         0.79%us-gaap_LongTermDebtPercentageBearingFixedInterestRate
/ us-gaap_DebtInstrumentAxis
= cost_PrivatePlacementNotesZeroPointSevenNinePercentDueMayTwentyTwentyFiveDomain
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember