v3.26.3
Cover Page - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2025
Feb. 13, 2026
Jun. 30, 2025
Cover [Abstract]      
Document Type 10-K/A    
Document Annual Report true    
Document Period End Date Dec. 31, 2025    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-38897    
Entity Registrant Name FASTLY, INC.    
Entity Incorporation, State or Country Code DE    
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    
Title of 12(b) Security Class A Common Stock, $0.00002 par value    
Trading Symbol FSLY    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction Flag false    
Entity Shell Company false    
Entity Public Float     $ 1,000,000
Entity Common Stock, Shares Outstanding   151,800,000  
Entity Central Index Key 0001517413    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Amendment Flag false