Condensed Consolidated Statements of Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (13,090)	$ (23,508)	$ (45,741)	$ (58,991)
Other comprehensive income (loss), net of tax impact:
Unrealized gain (loss) on available-for-sale securities	(282)	188	1,055	28
Foreign currency translation adjustments	47	(27)	137	(27)
Other comprehensive income (loss)	(235)	161	1,192	1
Comprehensive loss	(13,325)	(23,347)	(44,549)	(58,990)
Comprehensive gain (loss) attributable to redeemable noncontrolling interest	300	(950)	(4,700)	(950)
Comprehensive loss attributable to Guardant Health, Inc.	$ (13,025)	$ (24,297)	$ (49,249)	$ (59,940)