Condensed Consolidated Statements of Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (21,351)	$ (13,844)
Other comprehensive income (loss), net of tax impact:
Unrealized gain (loss) on available-for-sale securities	485	(298)
Foreign currency translation adjustments	(69)	0
Other comprehensive income (loss)	416	(298)
Comprehensive loss	(20,935)	(14,142)
Comprehensive loss attributable to redeemable noncontrolling interest	(4,700)	0
Comprehensive loss attributable to Guardant Health, Inc.	$ (25,635)	$ (14,142)