Net Loss Per Share Attributable to Guardant Health, Inc. Common Stockholders - Schedule of Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net loss	$ (11,300)	$ (21,351)	$ (21,639)	$ (13,844)	$ (32,651)	$ (35,483)
Fair value adjustment of redeemable noncontrolling interest	300		0		5,000	0
Net loss attributable to Guardant Health, Inc. common stockholders	$ (11,600)		$ (21,639)		$ (37,651)	$ (35,483)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (0.13)		$ (1.75)		$ (0.43)	$ (2.92)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	89,036		12,388		87,494	12,155