Investment in Joint Venture (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended
		May 31, 2018	Mar. 31, 2020	Dec. 31, 2019	Apr. 30, 2018
Schedule of Equity Method Investments [Line Items]
Assets	[1]		$ 948,167	$ 962,535
Threshold percentage of fair value that is no less than internal rate of return			20.00%
Fair value of joint venture threshold			40.00%
Guardant Health AMEA, Inc
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in joint venture		$ 9,000
Equity method investment, shares purchased		40,000
Equity method investment, ownership percentage		50.00%			50.00%
SoftBank | Guardant Health AMEA, Inc
Schedule of Equity Method Investments [Line Items]
Equity method investment, shares purchased		40,000
Equity method investment, ownership percentage		50.00%
Redeemable noncontrolling interest, redemption value		$ 41,000
Variable Interest Entity
Schedule of Equity Method Investments [Line Items]
Assets			$ 42,000	$ 45,100

[1] As of March 31, 2020 and December 31, 2019, includes $42.0 million and $45.1 million of assets, respectively, that can be used only to settle obligations of the consolidated variable interest entity (“VIE”) and VIE’s subsidiaries, and $4.6 million and $5.7 million of liabilities of the consolidated VIE and VIE’s subsidiaries, respectively, for which their creditors do not have recourse to the general credit of the Company. See Note 3, Investment in Joint Venture.