| Document And Entity Information | Jan. 06, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | DAILY JOURNAL CORPORATION |
| Document, Type | 8-K |
| Document, Period End Date | Jan. 6, 2021 |
| Entity, Incorporation, State or Country Code | SC |
| Entity, File Number | 0-14665 |
| Entity, Tax Identification Number | 95-4133299 |
| Entity, Address, Address Line One | 915 E. First Street |
| Entity, Address, City or Town | Los Angeles |
| Entity, Address, State or Province | CA |
| Entity, Address, Postal Zip Code | 90012 |
| City Area Code | 213 |
| Local Phone Number | 229-5300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | DJCO |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0000783412 |
