Name:	cost_PortionOfEachShareIssuedCountedTowardsLimitOfSharesAvailable
Namespace Prefix:	cost_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

Number of performance-based compensation awards outstanding at the end of the period.

No definition available.

Name:	cost_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingPerformanceBased
Namespace Prefix:	cost_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Number of performance-based compensation awards granted to executive officers upon achievement of the specified performance targets as approved by the Company's Board of Directors.

No definition available.

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Namespace Prefix:	cost_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The number of time-based compensation restricted stock units outstanding at the end of the period.

No definition available.

Name:	cost_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingTimeBased
Namespace Prefix:	cost_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Number of additional shares authorized for issuance under an established share-based compensation plan.

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901