Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company’s condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements include the accounts of Guardant Health, Inc. and its consolidated Joint Venture. Other stockholders’ interests in the Joint Venture are shown in the condensed consolidated financial statements as redeemable noncontrolling interests. All significant intercompany balances and transactions have been eliminated in consolidation. The Company believes that its existing cash and cash equivalents and marketable securities as of September 30, 2018 will be sufficient to allow the Company to fund its current operating plan through at least a period of one year after the date the condensed consolidated financial statements are issued. As the Company continues to incur losses, its transition to profitability is dependent upon a level of revenues adequate to support the Company’s cost structure. If the Company’s transition to profitability is not consistent with its current operating plan, the Company may have to seek additional capital.
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in conformity with GAAP requires management to make certain estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and the related disclosures at the date of the condensed consolidated financial statements, as well as the reported amounts of revenues and expenses during the periods presented. The Company bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. Estimates are used in several areas including, but not limited to, best estimate of selling price used in the accounting for multiple-element revenue arrangements, estimation of potential credit losses on accounts receivable, the valuation of inventory, recovery of long-lived assets, stock-based compensation, fair value of common stock and warrants, contingencies, the provision for income taxes, including related reserves, valuation of redeemable noncontrolling interest, among others. These estimates generally involve complex issues and require judgments, involve the analysis of historical results and prediction of future trends, can require extended periods of time to resolve and are subject to change from period to period. Actual results may differ materially from management’s estimates.
Unaudited Interim Condensed Financial Statements	Unaudited Interim Condensed Financial Statements The accompanying condensed consolidated balance sheet as of September 30, 2018, the condensed consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017, and cash flows for the nine months ended September 30, 2018 and 2017, and the related interim condensed consolidated disclosures are unaudited. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with GAAP for interim financial information and in accordance with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X of the Securities Act of 1933, as amended (the “Securities Act”). Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. These unaudited condensed consolidated financial statements include all adjustments, consisting only of normal recurring accruals that the Company believes are necessary to fairly state the financial position and the results of the Company’s operations and cash flows for interim periods in accordance with GAAP. Interim-period results are not necessarily indicative of results of operations or cash flows for a full year or any subsequent interim period. The accompanying condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements and notes thereto included in the Company’s Registration Statement on Form S-1 declared effective by the SEC on October 3, 2018.
JOBS Act Accounting Election	JOBS Act Accounting Election The Company is an “emerging growth company” within the meaning of the Jumpstart Our Business Act of 2012, or JOBS Act. Section 107(b) of the JOBS Act provides that an emerging growth company can leverage the extended transition period, provided in Section 102(b) of the JOBS Act, for complying with new or revised accounting standards. Thus, an emerging growth company can delay the adoption of new or revised accounting standards that have different effective dates for public and private companies until those standards apply to private companies. The Company has elected to use this extended transition period and, as a result, the financial statements may not be comparable to companies that comply with public company effective dates. The Company also intends to rely on other exemptions provided by the JOBS Act, including without limitation, not being required to comply with the auditor attestation requirements of Section 404(b) of the Sarbanes-Oxley Act of 2002.
Foreign Currency Translations	Foreign Currency Translation The functional currency of the branch offices of the consolidated Joint Venture is the local currency. The assets and liabilities of the subsidiaries are translated into U.S. dollars at exchange rates in effect at each balance sheet date, with the resulting translation adjustments recorded to a separate component of accumulated other comprehensive loss within stockholders’ equity. Income and expense accounts are translated at average exchange rates during the period. Foreign currency transaction gains and losses resulting from transactions denominated in a currency other than the functional currency are recognized in the consolidated statements of operations. For the three and nine months ended September 30, 2018, foreign currency translation adjustment was immaterial as the branch offices of the Company’s consolidated Joint Venture has limited operations.
Cash and Cash Equivalents and Restricted Cash	Cash and Cash Equivalents and Restricted Cash Cash equivalents consist of highly liquid investments with original maturities at the time of purchase of three months or less. Cash equivalents include bank demand deposits and money market accounts that invest primarily in U.S. government-backed securities and treasuries. Cash equivalents are carried at cost, which approximates their fair value.
Concentration of Risk	Concentration of Risk The Company is subject to credit risk from its portfolio of cash equivalents held at one commercial bank and investments in marketable securities. The Company limits its exposure to credit losses by investing in money market funds through a U.S. bank with high credit ratings. The Company’s cash may consist of deposits held with banks that may at times exceed federally insured limits, however, its exposure to credit risk in the event of default by the financial institution is limited to the extent of amounts recorded on the condensed consolidated balance sheets. The Company performs evaluations of the relative credit standing of these financial institutions to limit the amount of credit exposure. The Company also invests in investment‑grade debt instruments and has policy limits for the amount it can invest in any one type of security, except for securities issued or guaranteed by the U.S. government. The goals of the Company’s investment policy, in order of priority, are as follows: safety and preservation of principal and diversification of risk; liquidity of investments sufficient to meet cash flow requirements; and a competitive after‑tax rate of return. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, investment type and issuer, as a result, the Company is not exposed to any significant concentrations of credit risk from these financial instruments. The Company is also subject to credit risk from its accounts receivable. The majority of the Company’s accounts receivable arises from the provision of precision oncology services in the United States and are primarily with biopharmaceutical companies with high credit ratings. The Company has not experienced any material losses related to receivables from individual customers, or groups of customers. The Company does not require collateral. Accounts receivable are recorded at the invoiced amount and do not bear interest.
Accounts Receivable	Accounts Receivable Accounts receivable represent valid claims against biopharmaceutical companies, research institutes and international distributors. The Company evaluates the collectability of its accounts receivable and provides for an allowance for potential credit losses based on management’s best estimate of the amount of probable credit losses. As of September 30, 2018 and December 31, 2017, the Company had no allowance for doubtful accounts.
Revenue Recognition	Revenue Recognition The Company derives revenue from the provision of precision oncology testing services provided to its ordering physicians and biopharmaceutical customers, as well as from biopharmaceutical research and development services provided to its biopharmaceutical customers. Precision oncology services include genomic profiling and the delivery of other genomic information derived from the Company’s platform. Development services include the development of new platforms and information solutions, including companion diagnostic development and laboratory services. The Company currently receives payments from commercial third-party payers, certain hospitals and oncology centers and individual patients, as well as biopharmaceutical companies and research institutes. The Company recognizes revenue when all of the following criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery has occurred; (iii) the fee is fixed or determinable; and (iv) collectability is reasonably assured. Criterion (i) is satisfied when the Company has an arrangement or contract in place. Criterion (ii) is satisfied when the Company delivers a test report corresponding to each sample, without further commercial obligations. Determination of criteria (iii) and (iv) are based on management’s judgments regarding whether the fee is fixed or determinable, and whether the collectability of the fee is reasonably assured. The Company recognizes revenue from the sale of its precision oncology tests for clinical customers, including certain hospitals, cancer centers, other institutions and patients, at the time results of the test are reported to physicians, if criteria (i) through (iv) above are met. The Company recognizes revenue on a cash basis when it cannot conclude that criteria (iii) and (iv) have been met. Most of precision oncology tests requested by clinical customers are sold without a contracted engagement with a third-party payer; therefore, the Company experiences significant variability in collections and does not have sufficient history to establish a predictable pattern of payment. Because the price is not fixed or determinable and collectability is not reasonably assured, the Company recognizes revenue on a cash basis for sales of its liquid biopsy tests to clinical customers where collection depends on a third-party payer or the individual patient. The Company uses judgment in its assessment of whether the fee is fixed or determinable and whether collectability is reasonably assured in determining when to recognize revenue. Accordingly, the Company expects to recognize revenue on a cash basis for these clinical customers until it has sufficient history to reliably estimate payment patterns. In August 2018, the Company received positive coverage decision under Medicare and is in the process of evaluating whether this impacts its assessment of when revenue recognition criteria are satisfied for clinical customers with Medicare coverage. The Company’s precision oncology information services are delivered electronically, and as such there are no shipping or handling fees incurred by the Company or billed to customers. Revenue from sales of the Company’s tests to biopharmaceutical customers are based on a negotiated price per test or on the basis of an agreement to provide certain testing volume, data access or biopharmaceutical research and development services over a defined period. The Company recognizes revenue upon delivery of the test results, or over the period in which biopharmaceutical research and development services are provided, as appropriate. Multiple-element arrangements The Company performs development services for its biopharmaceutical customers utilizing its precision oncology information platform. Contracts with biopharmaceutical customers are primarily analyzed as multiple-element arrangements given the nature of the service deliverables. For development services performed, the Company is compensated in various ways, including (i) through non-refundable regulatory and other developmental milestone payments; and (ii) through royalty and sales milestone payments. The Company performs development services as part of its normal activities. The Company records these payments as development services revenue in the condensed consolidated statements of operations using a proportional performance model over the period which the unit of accounting is delivered or based on the level of effort expended to date over the total expected effort, whichever is considered the most appropriate measure of performance. For development of new products or services under these arrangements, costs incurred before technological feasibility is assured are included as research and development expenses in the Company’s condensed consolidated statements of operations, while costs incurred thereafter are recorded as cost of development services. The Company collaborates with pharmaceutical companies in the development and clinical trials of new drugs. As part of these collaborations, the Company provides services related to regulatory filings with the FDA to support companion diagnostic device submissions for the Company’s liquid biopsy panels. Under these collaborations the Company generates revenue from achievement of milestones, as well as provision of on-going support. These collaboration arrangements include no royalty obligations. For revenue arrangements with multiple deliverables, the Company evaluates each deliverable to determine whether it qualifies as a separate unit of accounting. This determination is based on whether the deliverable has stand-alone value to the customer and whether a general right of return exists. In assessing whether an item has standalone value, the Company considers factors such as the research, development and commercialization capabilities of a third party and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the other party in the arrangement can use the other deliverables for their intended purpose without the receipt of the remaining elements, whether the value of the deliverable is dependent on the undelivered items and whether there are other vendors that can provide the undelivered elements. The consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the relative selling price of each deliverable. The Company allocates the arrangement consideration following a hierarchy to determine the relative selling price to be used for allocating revenue to deliverables: (i) vendor-specific objective evidence of fair value (“VSOE”), (ii) third-party evidence of selling price (“TPE”), and (iii) best estimate of the selling price (“BESP”) if neither VSOE nor TPE is available. The Company typically uses BESP to estimate the selling price, since it generally does not have VSOE or TPE of selling price for its units of accounting under multiple-element arrangements. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting. The consideration allocated to each unit of accounting is recognized as the related goods or services are delivered, limited to the consideration that is not contingent upon future deliverables. The Company uses judgment in identifying the deliverables in its arrangements, assessing whether each deliverable is a separate unit of accounting and in determining the best estimate of selling price for certain deliverables. The Company also uses judgment in determining the period over which the deliverables are recognized in certain of its arrangements. Any amounts received that do not meet the criteria for revenue recognition are recorded as deferred revenue until such criteria are met. The Company performed laboratory installation and maintenance services for one of its customers as part of a multiple-element arrangement entered into in 2017. The Company recognized certain revenue from its construction service deliverables in a multiple-element collaboration arrangement based on the completed-contract method. This method was used as the Company determined that it did not have the basis for estimating performance under the contract. Other construction service deliverables under that contract were recognized under the percentage-of-completion method due to the Company’s ability to make reasonably dependable estimates of the extent of progress toward contract completion. Construction services were completed in March 2018. Milestones The Company recognizes payments that are contingent upon achievement of a substantive milestone in their entirety in the period in which the milestone is achieved. Milestones are defined as events that can only be achieved based on the Company’s performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered substantive milestones. Further, the amounts received must relate solely to prior performance, be reasonable relative to all of the deliverables and payment terms within the agreement and commensurate with the Company’s performance to achieve the milestone after commencement of the agreement. Any contingent payment that becomes payable upon achievement of events that are not considered substantive milestones are allocated to the units of accounting previously identified at the inception of an arrangement when the contingent payment is received and revenue is recognized based on the revenue recognition criteria for each unit of accounting. Revenue from commercial milestone payments are recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met.
Costs of Precision Oncology Testing	Costs of Precision Oncology Testing Cost of precision oncology testing generally consists of cost of materials, direct labor including bonus, benefit and stock-based compensation, equipment and infrastructure expenses associated with processing liquid biopsy test samples (including sample accessioning, library preparation, sequencing, quality control analyses and shipping charges to transport blood samples), freight, curation of test results for physicians and license fees due to third parties. Infrastructure expenses include depreciation of laboratory equipment, rent costs, amortization of leasehold improvements and information technology costs. Costs associated with performing the Company’s tests are recorded as the tests are processed regardless of whether revenue was recognized with respect to that test. Royalties for licensed technology calculated as a percentage of revenues generated using the associated technology are recorded as expense at the time the related revenues are recognized. One-time royalty payments related to signing of license agreements or other milestones, such as issuance of new patents, are amortized to expense over the expected useful life of the patents.
Cost of Development Services	Cost of Development Services Cost of development service includes costs incurred for the performance of development services requested by the Company’s customers. For development of new products, costs incurred before technological feasibility has been achieved are reported as research and development expenses, while costs incurred thereafter are reported as cost of development services.
Deferred Offering Costs	Deferred Offering Costs Deferred offering costs consist of fees and expenses incurred in connection with the anticipated sale of the Company’s common stock in the IPO, including the legal, accounting, printing and other IPO-related costs. Deferred offering costs of $4.3 million are capitalized and classified within noncurrent assets on the condensed consolidated balance sheet as of September 30, 2018. In October 2018, upon completion of the IPO, the Company reclassified these costs into additional paid-in capital as a reduction of the net proceeds received from the IPO. During the nine months ended September 30, 2018, $0.2 million of the deferred offering costs were paid.
Stock-Based Compensation	Stock‑Based Compensation Stock‑based compensation related to stock options granted to the Company’s employees is measured at the grant date based on the fair value of the award. The fair value is recognized as expense over the requisite service period, which is generally the vesting period of the respective awards. Starting January 1, 2017, upon adoption of Accounting Standards Update (“ASU”) 2016 -09, Compensation – Stock Compensation (Topic 718), forfeitures are accounted for as they occur. The Company uses the Black‑Scholes option‑pricing model to estimate the fair value of its stock options. The Black-Scholes option-pricing model requires assumptions to be made related to the estimated fair value of the Company’s common stock at the applicable measurement date, expected term of an award, expected volatility, risk-free rate and expected dividend yield. The Company accounts for stock options issued to non-employees based on the estimated fair value at the grant date and re-measured at each reporting period using the Black-Scholes option-pricing model. The measurement of stock-based compensation is subject to periodic adjustments as the underlying equity instruments vest, and the resulting change in value, if any, is recognized in the Company’s condensed consolidated statements of operations during the period that the related services are rendered.
Net Loss Per Share Attributable to Common Shareholders	Net Loss Per Share Attributable to Common Stockholders The Company calculates its basic and diluted net loss per share attributable to common stockholders in conformity with the two-class method required for companies with participating securities. The Company considers its convertible preferred stock to be participating securities. In the event a dividend is declared or paid on the Company’s common stock, holders of preferred stock are entitled to a share of such dividend in proportion to the holders of common stock on an as-if converted basis. Under the two-class method, basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Net loss attributable to common stockholders is determined by allocating undistributed earnings between common and preferred stockholders. The diluted net loss per share attributable to common stockholders is computed by giving effect to all potential dilutive common stock equivalents outstanding for the period determined using the treasury stock method. The net loss attributable to common stockholders was not allocated to the preferred stock under the two-class method as the preferred stock does not have a contractual obligation to share in the Company’s losses. For purposes of this calculation, convertible preferred stock, common stock warrants and stock options are considered common stock equivalents but have been excluded from the calculation of diluted net loss per share attributable to common stockholders as their effect is anti-dilutive.
Recent Accounting Pronouncements Not Yet Adopted	Recent Accounting Pronouncements Not Yet Adopted In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASC606”). The new standard is based on the principle that revenue should be recognized to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Since its initial release, the FASB has issued several amendments to the standard, which include clarification of accounting guidance related to identification of performance obligations, intellectual property licenses and principal vs. agent considerations. The new guidance and all subsequent amendments will be effective for the Company beginning on January 1, 2019 and may be applied using either the full retrospective method, in which case the standard would be applied to each prior reporting period presented, or the modified retrospective method, in which case the cumulative effect of applying the standard would be recognized at the date of initial application. Early adoption is permitted. The Company identified certain differences in accounting for revenue recognition as a result of developing an adoption plan for ASC 606. For precision oncology testing revenue, the Company identified a difference in accounting for certain revenue arrangements from the application of the new revenue accounting standard as compared to the previous revenue accounting standards. Historically, for certain clinical customers, the Company deferred revenue recognition until cash receipt when the price pursuant to the underlying customer arrangement was not fixed and determinable and collectability was not reasonably assured. Under the new standard, this is considered variable consideration. For these arrangements, the Company will record an estimate of the transaction price, subject to the constraint in the new standard for variable consideration, as revenue at the time of delivery. This estimate will be monitored in subsequent periods and adjusted as necessary based on actual collection experience. This will result in earlier revenue recognition as compared to previous revenue recognition. The Company is still in the process of quantifying the impact of new guidance on its condensed consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The new standard requires lessees to recognize most leases on their balance sheets as lease liabilities with corresponding right-of-use assets and eliminates certain real estate-specific provisions. The new guidance will be effective for the Company beginning in 2020, at which time, the new guidance will be adopted on a modified retrospective transition basis for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the consolidated financial statements. The Company is currently evaluating the impact of new guidance on its condensed consolidated financial statements and anticipates the recognition of additional assets and corresponding liabilities on its condensed consolidated balance sheet related to leases. The adoption of the standard is also expected to materially impact the Company’s condensed consolidated financial statement disclosures related to leases. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments, which amends the impairment model by requiring entities to use a forward-looking approach based on expected losses to estimate credit losses on certain types of financial instruments, including trade receivables and available for sale debt securities. The new guidance is effective for the Company beginning in 2021, with early adoption permitted. The Company is currently evaluating the impact of the new guidance on its condensed consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. The new guidance is effective for the Company beginning in 2019, with early adoption permitted. The Company is currently evaluating the impact of the new guidance on its condensed consolidated financial statements. On December 22, 2017, the U.S. federal government enacted the Tax Cuts and Jobs Act (“the Tax Act”). The Tax Act contains, among other things, significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of 21% for tax years beginning after December 31, 2017, limitation of the deduction for net operating losses to 80% of current year taxable income and elimination of net operating loss carrybacks, implementing a territorial tax system, and requiring a mandatory one-time tax on U.S. owned undistributed foreign earnings and profits known as the transition tax. In December 2017, SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”) to address the accounting implications of recently enacted U.S. federal tax reform. SAB 118 allows companies to record provisional amounts during a measurement period not to extend beyond one year of the enactment date to address ongoing guidance and tax interpretations that are expected over the next 12 months. The Company has adopted SAB 118 and currently considers its accounting of the impact of U.S. federal tax reform to be incomplete but continues to make a reasonable estimate of the effects on our existing deferred tax assets. The Company expects to complete the remainder of the analysis within the measurement period in accordance with SAB 118. Adjustments, if any, are not expected to impact the statement of operations due to the full valuation allowance on the Company’s deferred tax assets.