| Document And Entity Information | Oct. 07, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | Assembly Biosciences, Inc. |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Central Index Key | 0001426800 |
| Document Period End Date | Oct. 19, 2020 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-35005 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 20-8729264 |
| Entity Address, Address Line One | 331 Oyster Point Blvd., Fourth Floor |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 833 |
| Local Phone Number | 509-4583 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of each class | Common Stock, par value $0.001 |
| Trading Symbol | ASMB |
| Name of each exchange on which registerd | NASDAQ |
