Condensed Consolidated Statements of Operations (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue:
Precision oncology testing	$ 18,298	$ 10,253	$ 50,311	$ 27,927
Development services	3,394	879	7,455	1,913
Total revenue	21,692	11,132	57,766	29,840
Costs and operating expenses:
Cost of precision oncology testing	9,671	7,603	27,222	20,928
Cost of development services	380	1,058	2,041	1,542
Research and development expense	14,253	7,246	34,062	17,442
Sales and marketing expense	13,464	7,808	36,351	22,941
General and administrative expense	8,129	16,095	23,645	27,982
Total costs and operating expenses	45,897	39,810	123,321	90,835
Loss from operations	(24,205)	(28,678)	(65,555)	(60,995)
Interest income	958	657	2,932	1,222
Interest expense	(304)	(303)	(952)	(2,398)
Loss on debt extinguishment	0	0	0	(5,075)
Other income (expense), net	43	(266)	4,587	(915)
Loss before provision for income taxes	(23,508)	(28,590)	(58,988)	(68,161)
Provision for income taxes	0	0	3	0
Net loss	(23,508)	(28,590)	(58,991)	(68,161)
Fair value adjustment of redeemable noncontrolling interest	(950)	0	(950)	0
Net loss attributable to Guardant Health, Inc.	(24,458)	(28,590)	(59,941)	(68,161)
Deemed dividend related to repurchase of Series A convertible preferred stock			(1,100)
Net loss attributable to Guardant Health, Inc. common stockholders	$ (24,458)	$ (33,306)	$ (59,941)	$ (73,935)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (1.94)	$ (2.76)	$ (4.87)	$ (5.76)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	12,582	12,073	12,300	12,831
Series A Convertible Preferred Stock
Costs and operating expenses:
Deemed dividend related to repurchase of Series A convertible preferred stock	$ 0	$ (4,716)	$ 0	$ (4,716)
Series D Convertible Preferred Stock
Costs and operating expenses:
Deemed dividend related to repurchase of Series A convertible preferred stock	$ 0	$ 0	$ 0	$ (1,058)