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	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net loss	$ (67,851)	$ (84,263)	$ (83,221)
Adjustment of redeemable noncontrolling interest	(7,800)	(800)	0
Net loss attributable to Guardant Health, Inc.	(75,651)	(85,063)	(83,221)
Deemed dividend related to repurchase of convertible preferred stock	(1,100)
Net loss attributable to Guardant Health, Inc. common stockholders	$ (75,651)	$ (85,063)	$ (88,995)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (0.84)	$ (2.80)	$ (7.07)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	90,597	30,403	12,582
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	$ 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	$ (1,058)