Description of Business (Details) $ / shares in Units, $ in Thousands			1 Months Ended		12 Months Ended
	Oct. 09, 2018 USD ($) $ / shares shares	Sep. 19, 2018	May 31, 2019 USD ($) $ / shares shares	Sep. 30, 2018 shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 shares	May 31, 2018	Apr. 30, 2018
Subsidiary, Sale of Stock [Line Items]
Stock split, conversion ratio		0.7378		0.7378
Common stock, shares authorized (in shares)				350,000,000	350,000,000	350,000,000
Convertible preferred stock, shares authorized (in shares)				10,000,000	10,000,000	10,000,000
Proceeds from initial public offering and follow on offering | $	$ 249,500
Common stock, shares reserved for future issuance (in shares)					7,911,031	12,067,162
Issuance of common stock upon follow-on offering, net of offering costs (in shares)			5,175,000
Share price of stock issued (in usd per share) | $ / shares			$ 71.00
Issuance of common stock upon follow-on offering, net of offering costs of $723 | $			$ 349,700		$ 349,709
IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	14,375,000
Sale of stock, price (usd per share) | $ / shares	$ 19.00
Proceeds from initial public offering and follow on offering | $	$ 249,500
Issuance of common stock upon follow-on offering, net of offering costs (in shares)	14,375,000
Underwriters Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Issuance of common stock upon follow-on offering, net of offering costs (in shares)			675,000
Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)	58,264,577
Guardant Health AMEA, Inc
Subsidiary, Sale of Stock [Line Items]
Equity method investment, ownership percentage							50.00%	50.00%
Warrants, Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)	7,636