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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net loss	$ (49,739)	$ (11,300)	$ (81,568)	$ (32,651)
Adjustment of redeemable noncontrolling interest	(4,900)	(300)	(800)	(5,000)
Net loss attributable to Guardant Health, Inc. common stockholders	$ (54,639)	$ (11,600)	$ (82,368)	$ (37,651)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (0.57)	$ (0.13)	$ (0.87)	$ (0.43)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	96,011	89,036	95,196	87,494