Condensed Consolidated Statements of Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (11,300)	$ (21,639)	$ (32,651)	$ (35,483)
Other comprehensive income (loss), net of tax impact:
Unrealized gain (loss) on available-for-sale securities	852	138	1,337	(160)
Foreign currency translation adjustments	159	0	90	0
Other comprehensive income (loss)	1,011	138	1,427	(160)
Comprehensive loss	(10,289)	(21,501)	(31,224)	(35,643)
Comprehensive loss attributable to redeemable noncontrolling interest	(300)	0	(5,000)	0
Comprehensive loss attributable to Guardant Health, Inc.	$ (10,589)	$ (21,501)	$ (36,224)	$ (35,643)