| Cover Page | Oct. 28, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Document Period End Date | Oct. 28, 2020 |
| Entity Registrant Name | FASTLY, INC. |
| Entity Central Index Key | 0001517413 |
| Amendment Flag | false |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38897 |
| Entity Tax Identification Number | 27-5411834 |
| Entity Address, Address Line One | 475 Brannan Street, Suite 300 |
| Entity Address, City or Town | San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94107 |
| City Area Code | 844 |
| Local Phone Number | 432-7859 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Class A Common Stock, $0.00002 par value |
| Trading Symbol | “FSLY” |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
