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, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net loss	$ (21,351)	$ (13,844)
Fair value adjustment of redeemable noncontrolling interest	4,700	0
Net loss attributable to Guardant Health, Inc. common stockholders	$ (26,051)	$ (13,844)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (0.30)	$ (1.16)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	85,935	11,920