Condensed Consolidated Statements of Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net loss	$ (23,508)	$ (28,590)	$ (58,991)	$ (68,161)
Other comprehensive loss, net of tax impact:
Unrealized loss on available-for-sale securities	188	6	28	(55)
Foreign currency translation adjustments	(27)	0	(27)	0
Other comprehensive loss	161	6	1	(55)
Comprehensive loss	(23,347)	(28,584)	(58,990)	(68,216)
Comprehensive loss attributable to redeemable noncontrolling interest	(950)	0	(950)	0
Comprehensive loss attributable to Guardant Health, Inc.	$ (24,297)	$ (28,584)	$ (59,940)	$ (68,216)