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Share-based Compensation Arrangement by Share-based Payment Award, Holding Period During Vesting

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Share-based Compensation Arrangement by Share-based Payment Award, Share Price Goal Period

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Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross

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Weighted Average Number of Shares, Common Stock Subject to Repurchase or Cancellation, Per Share

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Aggregate number of common shares reserved for future issuance.

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Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements payable after one year (or the operating cycle, if longer).

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Amount of cost to be recognized for option under share-based payment arrangement.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Restrictions on equity-based instruments during the vesting period, such as the inability to transfer unvested awards, are not taken into account in estimating the fair value of the award. However, restrictions that remain in effect after an award is vested, such as the inability to transfer or hedge vested options or a prohibition on the sale of outstanding vested shares (or other type of equity) for a period of time, affect the estimate of an award's fair value.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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Number of additional shares authorized for issuance under share-based payment arrangement.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Purchase price of common stock expressed as a percentage of its fair value.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended		12 Months Ended
	May 26, 2020	Jan. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value, grants in period (in usd per share)			$ 49.73	$ 55.16	$ 46.85	$ 53.02
Stock based compensation not recognized, options			$ 21,000		$ 21,000
Stock based compensation not recognized, period for recognition (years)					2 years 6 months
Total market-based restricted stock units approved and granted					114,472
Stock-based compensation			$ 55,198	$ 5,484	$ 87,351	$ 11,882
Common stock, shares reserved for future issuance (in shares)			8,277,673		8,277,673		8,407,162
Class B \| AMEA 2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value, grants in period (in usd per share)			$ 0.33		$ 0.33
Stock based compensation not recognized, options			$ 700		$ 700
Stock based compensation not recognized, period for recognition (years)					2 years 4 months 24 days
Total market-based restricted stock units approved and granted					3,902,224
Shares authorized (in shares)					4,595,555
Guardant Health AMEA, Inc \| Class B \| AMEA 2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares reserved for future issuance (in shares)			4,595,555		4,595,555
Guardant Health AMEA, Inc \| Class B \| AMEA 2020 Plan \| Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Market-based stock unit vesting period					4 years
Option term, expiration period					10 years
Guardant Health AMEA, Inc \| Class B \| AMEA 2020 Plan \| Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price of common stock (as a percent of the fair value of common stock)					110.00%
Option term, expiration period					5 years
Threshold percentage for individual's combined voting power triggering five year option term					10.00%
Stock options issued and outstanding
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercises in period, intrinsic value			$ 46,800	$ 75,300	$ 97,100	$ 191,700
Common stock, shares reserved for future issuance (in shares)			3,356,774		3,356,774		4,494,889
Expected dividend yield			0.00%	0.00%	0.00%	0.00%
Stock options issued and outstanding \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility			68.40%	64.60%	73.30%	68.30%
Risk-free interest rate			0.40%	1.90%	1.60%	2.70%
Stock options issued and outstanding \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility			63.60%	63.20%	63.60%	63.20%
Risk-free interest rate			0.30%	1.60%	0.30%	1.60%
Shares underlying unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation not recognized, period for recognition (years)					3 years 4 months 24 days
Stock based compensation not recognized, other than options			$ 77,100		$ 77,100
Common stock, shares reserved for future issuance (in shares)			949,276		949,276		496,131
ESPP shares granted (in shares)					601,451
MSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation not recognized, period for recognition (years)					1 year 1 month 6 days
Stock based compensation not recognized, other than options			$ 162,100		$ 162,100
Total market-based restricted stock units approved and granted					3,391,148
Market-based restricted stock share price goal	30 days
Market-based stock unit vesting period	7 years
Weighted average grant date fair value of MSU (in usd per share)					$ 67.00
Stock-based compensation			46,800		$ 65,100
Expected dividend yield					0.00%
Expected volatility					65.50%
Risk-free interest rate					0.53%
Risky rate, cost of equity					0.16
MSUs \| Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total market-based restricted stock units approved and granted	1,695,574
MSUs \| President and Chief Operating Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total market-based restricted stock units approved and granted	1,695,574
MSUs \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Market-based stock units holding period during vesting	12 months
Weighted-average derivative service period					2 years 25 days
Discount for post-vesting restrictions					14.50%
MSUs \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Market-based stock units holding period during vesting	6 months
Weighted-average derivative service period					9 months 29 days
Discount for post-vesting restrictions					10.40%
Shares available for issuance under the 2018 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation not recognized, period for recognition (years)					1 month 6 days
Stock based compensation not recognized, other than options			400		$ 400
Stock-based compensation			$ 700	$ 800	$ 2,300	$ 1,800
Purchase price of common stock (as a percent of the fair value of common stock)					85.00%
Common stock, shares reserved for future issuance (in shares)			1,574,367		1,574,367		689,917	922,250
Expected dividend yield				0.00%	0.00%	0.00%
Expected volatility				60.30%	73.20%
Risk-free interest rate				2.10%	0.20%
Shares authorized (in shares)		942,614
Maximum employee subscription rate, ESPP			10.00%		10.00%
Common stock issued under employee stock purchase plan (in shares)			0	93,203	58,164	212,905
ESPP shares granted (in shares)			0
Shares available for issuance under the 2018 Employee Stock Purchase Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility						60.30%
Risk-free interest rate						2.50%
Shares available for issuance under the 2018 Employee Stock Purchase Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility						60.20%
Risk-free interest rate						2.10%
Employee Stock Option, Early Exercise
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock subject to repurchase (in shares)					15,681		23,981
Stock subject to repurchase, per share (in usd per share)			$ 4.66		$ 4.66		$ 4.66
Cash proceeds received for unvested shares			$ 100		$ 100		$ 100