Cover Page - shares	3 Months Ended
	Mar. 31, 2020	May 05, 2020
Entity Listings [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
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	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Central Index Key	0001517413
Current Fiscal Year End Date	--03-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false
Common Class A
Entity Listings [Line Items]
Entity Common Stock, Shares Outstanding		72,700,000
Common Class B
Entity Listings [Line Items]
Entity Common Stock, Shares Outstanding		23,600,000