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Debt (Carrying Value and Estimated Fair Value of Company's Long Term Debt) (Details) (USD $) In Millions, unless otherwise specified
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6 Months Ended |
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Feb. 15, 2015
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Aug. 31, 2014
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| Debt Instrument [Line Items] |
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| Total long-term debt, carrying value |
$ 5,030us-gaap_LongTermDebt |
$ 5,093us-gaap_LongTermDebt |
| Less current portion, carrying value |
1,200us-gaap_LongTermDebtCurrent |
0us-gaap_LongTermDebtCurrent |
| Long-term debt, excluding current maturities |
3,830us-gaap_LongTermDebtNoncurrent |
5,093us-gaap_LongTermDebtNoncurrent |
| Total long-term debt, fair value |
5,143us-gaap_LongTermDebtFairValue |
5,217us-gaap_LongTermDebtFairValue |
| Less current portion, fair value |
1,203cost_LongtermDebtFairValueCurrentMaturities |
0cost_LongtermDebtFairValueCurrentMaturities |
| Long-term debt, excluding current portion, fair value |
3,940cost_LongtermDebtFairValueNonCurrent |
5,217cost_LongtermDebtFairValueNonCurrent |
| 0.65% Senior Notes due December 2015 |
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| Debt Instrument [Line Items] |
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| Debt instrument, stated interest rate (percent) |
0.65%us-gaap_LongTermDebtPercentageBearingFixedInterestRate / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesZeroPointSixFiveZeroPercentDueDecemberSevenTwentyFifteenMember |
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| Debt instrument, principal due date |
2015-12 |
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| Total long-term debt, carrying value |
1,200us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesZeroPointSixFiveZeroPercentDueDecemberSevenTwentyFifteenMember |
1,199us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesZeroPointSixFiveZeroPercentDueDecemberSevenTwentyFifteenMember |
| Total long-term debt, fair value |
1,203us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesZeroPointSixFiveZeroPercentDueDecemberSevenTwentyFifteenMember |
1,203us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesZeroPointSixFiveZeroPercentDueDecemberSevenTwentyFifteenMember |
| 5.5% Senior Notes due March 2017 |
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| Debt Instrument [Line Items] |
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| Debt instrument, stated interest rate (percent) |
5.50%us-gaap_LongTermDebtPercentageBearingFixedInterestRate / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesFivePointFivePercentDueMarchTwentySeventeenMember |
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| Debt instrument, principal due date |
2017-03 |
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| Total long-term debt, carrying value |
1,099us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesFivePointFivePercentDueMarchTwentySeventeenMember |
1,099us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesFivePointFivePercentDueMarchTwentySeventeenMember |
| Total long-term debt, fair value |
1,201us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesFivePointFivePercentDueMarchTwentySeventeenMember |
1,223us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesFivePointFivePercentDueMarchTwentySeventeenMember |
| 1.125% Senior Notes due December 2017 |
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| Debt Instrument [Line Items] |
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| Debt instrument, stated interest rate (percent) |
1.125%us-gaap_LongTermDebtPercentageBearingFixedInterestRate / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointOneTwoFivePercentDueDecemberFifteenTwentySeventeenMember |
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| Debt instrument, principal due date |
2017-12 |
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| Total long-term debt, carrying value |
1,100us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointOneTwoFivePercentDueDecemberFifteenTwentySeventeenMember |
1,100us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointOneTwoFivePercentDueDecemberFifteenTwentySeventeenMember |
| Total long-term debt, fair value |
1,102us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointOneTwoFivePercentDueDecemberFifteenTwentySeventeenMember |
1,095us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointOneTwoFivePercentDueDecemberFifteenTwentySeventeenMember |
| 1.7% Senior Notes due December 2019 |
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| Debt Instrument [Line Items] |
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| Debt instrument, stated interest rate (percent) |
1.70%us-gaap_LongTermDebtPercentageBearingFixedInterestRate / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointSevenPercentDueDecemberTwentyNineteenMember |
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| Debt instrument, principal due date |
2019-12 |
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| Total long-term debt, carrying value |
1,198us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointSevenPercentDueDecemberTwentyNineteenMember |
1,198us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointSevenPercentDueDecemberTwentyNineteenMember |
| Total long-term debt, fair value |
1,194us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointSevenPercentDueDecemberTwentyNineteenMember |
1,186us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_SeniorNotesOnePointSevenPercentDueDecemberTwentyNineteenMember |
| Other Long Term Debt |
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| Debt Instrument [Line Items] |
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| Total long-term debt, carrying value |
433us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_OtherLongTermDebtMember |
497us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = cost_OtherLongTermDebtMember |
| Total long-term debt, fair value |
$ 443us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_OtherLongTermDebtMember |
$ 510us-gaap_LongTermDebtFairValue / us-gaap_LongtermDebtTypeAxis = cost_OtherLongTermDebtMember |