| Cover Page - USD ($) | 12 Months Ended | ||
| Dec. 31, 2019 | Apr. 10, 2020 | Jun. 30, 2019 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Document Annual Report | true | ||
| Document Period End Date | Dec. 31, 2019 | ||
| Document Transition Report | false | ||
| Entity File Number | 001-33304 | ||
| Entity Registrant Name | FINJAN HOLDINGS, INC. | ||
| Entity Incorporation, State or Country Code | DE | ||
| Entity Tax Identification Number | 20-4075963 | ||
| Entity Address, Address Line One | 2000 University Avenue, Suite 600 | ||
| Entity Address, City or Town | East Palo Alto | ||
| Entity Address, State or Province | CA | ||
| Entity Address, Postal Zip Code | 94303 | ||
| City Area Code | 650 | ||
| Local Phone Number | 282-3228 | ||
| Title of 12(b) Security | Common Stock, par value $0.0001 per share | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Voluntary Filers | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Filer Category | Non-accelerated Filer | ||
| Entity Small Business | true | ||
| Entity Emerging Growth Company | false | ||
| Entity Shell Company | false | ||
| Entity Public Float | $ 47,486,164 | ||
| Entity Common Stock, Shares Outstanding | 27,700,242 | ||
| Documents Incorporated by Reference | N | ||
| Entity Central Index Key | 0001366340 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Document Fiscal Year Focus | 2019 | ||
| Document Fiscal Period Focus | FY | ||
| Amendment Flag | false | ||
| No Trading Symbol Flag | false |
