| Document And Entity Information | Nov. 10, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 10, 2021 |
| Entity Registrant Name | Allena Pharmaceuticals, Inc. |
| Entity Central Index Key | 0001624658 |
| Entity Emerging Growth Company | true |
| Entity File Number | 001-38268 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 45-2729920 |
| Entity Address, Address Line One | One Newton Executive Park |
| Entity Address, Address Line Two | Suite 202 |
| Entity Address, City or Town | Newton |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02462 |
| City Area Code | (617) |
| Local Phone Number | 467-4577 |
| Entity Information, Former Legal or Registered Name | Not Applicable |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Ex Transition Period | true |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | ALNA |
| Security Exchange Name | NASDAQ |
