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						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net loss	$ (13,090)	$ (11,300)	$ (21,351)	$ (23,508)	$ (21,639)	$ (13,844)	$ (45,741)	$ (58,991)
Fair value adjustment of redeemable noncontrolling interest	300			(950)			(4,700)	(950)
Net loss attributable to Guardant Health, Inc. common stockholders	$ (12,790)			$ (24,458)			$ (50,441)	$ (59,941)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (0.14)			$ (1.94)			$ (0.56)	$ (4.87)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	93,303			12,582			89,452	12,300