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
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net loss	$ (31,829)	$ (21,351)
Adjustment of redeemable noncontrolling interest	4,100	(4,700)
Net loss attributable to Guardant Health, Inc. common stockholders	$ (27,729)	$ (26,051)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (0.29)	$ (0.30)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	94,382	85,935