Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Oct. 31, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	001-38683
Entity Registrant Name	GUARDANT HEALTH, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	45-4139254
Entity Address, Address Line One	505 Penobscot Dr.
Entity Address, City or Town	Redwood City
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94063
City Area Code	855
Local Phone Number	698-8887
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
Entity Shell Company	false
Title of 12(b) Security	Common Stock, par value $0.00001
Trading Symbol	GH
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		99,972,386
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001576280
Current Fiscal Year End Date	--12-31