Net Loss Per Share Attributable to Guardant Health, Inc. Common Stockholders - Schedule of Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net loss	$ (23,508)	$ (28,590)	$ (58,991)	$ (68,161)
Fair value adjustment of redeemable noncontrolling interest	950	0	950	0
Net loss attributable to Guardant Health, Inc.	(24,458)	(28,590)	(59,941)	(68,161)
Deemed dividend related to repurchase of convertible preferred stock			(1,100)
Net loss attributable to Guardant Health, Inc. common stockholders	$ (24,458)	$ (33,306)	$ (59,941)	$ (73,935)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (1.94)	$ (2.76)	$ (4.87)	$ (5.76)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	12,582	12,073	12,300	12,831
Series A Convertible Preferred Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Deemed dividend related to repurchase of convertible preferred stock	$ 0	$ (4,716)	$ 0	$ (4,716)
Series D Convertible Preferred Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Deemed dividend related to repurchase of convertible preferred stock	$ 0	$ 0	$ 0	$ (1,058)