| Document and Entity Information | May 28, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | COSTCO WHOLESALE CORP /NEW |
| Amendment Flag | false |
| Entity Central Index Key | 0000909832 |
| Document Type | 8-K |
| Document Period End Date | May 28, 2020 |
| Entity Incorporation State Country Code | WA |
| Entity File Number | 0-20355 |
| Entity Tax Identification Number | 91-1223280 |
| Entity Address, Address Line One | 999 Lake Drive |
| Entity Address, City or Town | Issaquah |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98027 |
| City Area Code | 425 |
| Local Phone Number | 313-8100 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $.01 per share |
| Trading Symbol | COST |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
