Aggregate number of common shares reserved for future issuance.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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-Publisher FASB
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The cash inflow associated with the amount received from entity's first offering of stock to the public.

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The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

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Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

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Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Description of Business - Narrative (Details) $ / shares in Units, $ in Millions	1 Months Ended
	Oct. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2018 shares	May 31, 2018	Apr. 30, 2018	Dec. 31, 2017 shares
Subsidiary, Sale of Stock [Line Items]
Stock split, conversion ratio		0.7378
Common stock, shares reserved for future issuance		66,539,405			67,675,796
Guardant Health AMEA, Inc
Subsidiary, Sale of Stock [Line Items]
Equity method investment, ownership percentage			50.00%	50.00%
Subsequent Event \| IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction	14,375,000
Sale of stock, price per share \| $ / shares	$ 19.00
Net proceeds from issuance initial public offering \| $	$ 249.5
Common stock, shares reserved for future issuance	7,636
Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Common stock, shares reserved for future issuance		58,264,577			58,264,577
Convertible Preferred Stock \| Subsequent Event \| IPO
Subsidiary, Sale of Stock [Line Items]
Common stock, shares reserved for future issuance	58,264,577