| Document and Entity Information | Nov. 08, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001783183 |
| Document Type | 8-K |
| Document Period End Date | Nov. 8, 2021 |
| Entity Registrant Name | PHATHOM PHARMACEUTICALS, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-39094 |
| Entity Tax Identification Number | 82-4151574 |
| Entity Address, Address Line One | 100 Campus Drive |
| Entity Address, Address Line Two | Suite 102 |
| Entity Address, City or Town | Florham Park |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 07932 |
| City Area Code | (877) |
| Local Phone Number | 742-8466 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.0001 per share |
| Trading Symbol | PHAT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
