Investment in Joint Venture	9 Months Ended
Sep. 30, 2018
Equity Method Investments and Joint Ventures [Abstract]
Investment in Joint Venture	Investment in Joint Venture Variable Interest Entity (“VIE”) In connection with SoftBank’s purchase of its Series E convertible preferred stock, the Company entered into a joint venture agreement with an entity affiliated with SoftBank, a related party. In May 2018, the Company and SoftBank formed and capitalized Guardant Health AMEA, Inc. (the “Joint Venture”) for the sale, marketing and distribution of the Company’s tests in all areas worldwide, outside of North America, Central America, South America, the United Kingdom, all other member states of the European Union as of May 2017, Iceland, Norway, Switzerland and Turkey. The Company expects to rely on the Joint Venture to accelerate commercialization of its products in Asia, the Middle East and Africa, with an initial focus on Japan. Under the terms of the joint venture agreement, the Company paid $9.0 million for 40,000 shares of common stock, or 50% ownership interest, of the Joint Venture, and the affiliate of SoftBank contributed $41.0 million for 40,000 shares of common stock, or the other 50% ownership interest, of the Joint Venture. Neither party has the obligation to provide additional financial support to the Joint Venture. Each party holds two seats on the board of the Joint Venture and has to cast at least one vote for any board resolution of the Joint Venture to pass. The representatives of the Company on the Joint Venture’s board of directors have the right to appoint and remove a chief executive officer and a legal representative for the Joint Venture, in each case, subject to the approval of the full Joint Venture board of directors. The Joint Venture’s board of directors has the right to appoint and remove all other members of the Joint Venture’s senior management reporting to its chief executive officer and to approve the compensation of all foregoing individuals, including the compensation of the chief executive officer and legal representative. At the inception of the arrangement and at the end of each reporting period, the Company assesses whether the Joint Venture is a variable interest entity (“VIE”), and if so, who is the primary beneficiary of the VIE. As of September 30, 2018, the Company and SoftBank had equal ownership interests and equal voting rights in the Joint Venture, and the Joint Venture’s board consisted of an equal number of directors representing the interest of the Company and SoftBank, respectively. As of September 30, 2018, the Joint Venture’s board had the right to vote on all critical matters that most significantly impact the Joint Venture’s economic performance, except that the Company had the unilateral right to make pricing decisions. As of September 30, 2018, the Company had responsibility for the Joint Venture’s daily operations, while SoftBank served as a financing partner. The Company also entered into various ancillary agreements with the Joint Venture necessary to operate its business. The Joint Venture is deemed to be a VIE and considering the power and benefits criterion, the Company and SoftBank, collectively as a related party group, has the characteristics of the primary beneficiary of the Joint Venture as the related party group has the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance and has the obligation to absorb losses or the right to receive benefits from the VIE that could potentially be significant to the VIE. Because the Company is most closely associated with the Joint Venture within the related party group, it has been identified as the VIE’s primary beneficiary. As the primary beneficiary, the Company has consolidated the financial position, results of operations and cash flows of the Joint Venture in its financial statements and all intercompany balances have been eliminated in consolidation. The Company concluded the Joint Venture did not meet the definition of a business upon consolidation as it lacked the processes required to generate outputs. Upon consolidation no liabilities were assumed and other than cash, and any identifiable assets were related to intellectual property rights that the Company transferred to the Joint Venture shortly before it became its primary beneficiary and therefore such transfer was treated as a common control transaction. Upon consolidation, the non-controlling interest of the affiliate of SoftBank was recorded at its estimated fair value of $41.0 million, which is equal to the original investment made by the affiliate of SoftBank. As of September 30, 2018, the Joint Venture had total assets of approximately $49.5 million, which was primarily comprised of cash and security deposits. Although the Company consolidates the Joint Venture, the legal structure of the Joint Venture limits the recourse that its creditors will have over the Company’s general credit or assets. Similarly, the assets held in the Joint Venture can be used only to settle obligations of the Joint Venture. As of September 30, 2018, the Company has not provided financial or other support to the Joint Venture that was not previously contracted or required. Put-call arrangements The joint venture agreement includes a put-call arrangement with respect to the shares of the Joint Venture held by SoftBank and its affiliates. Under certain specified circumstances and on terms specified in the joint venture agreement, SoftBank will have the right to cause the Company to purchase all shares of the Joint Venture held by SoftBank and its affiliates, or the put right, and the Company will have a similar right to purchase all such shares, or the call right. If the Company’s business model were to change such that the sale, marketing and distribution of its tests in the territory covered by the joint venture agreement was no longer economical, SoftBank would have the right to cause the Company to purchase, or the Company would have the right to purchase, all of the shares of the Joint Venture held by SoftBank and its affiliates. In this instance, the Company would be required to repurchase the shares at an aggregate purchase price of $41.0 million, the original purchase price paid by SoftBank to the Joint Venture for the shares. Additionally, both the Company and SoftBank may exercise its respective put-call rights for the Company to purchase all shares of the Joint Venture held by SoftBank in the event of (i) certain material disagreement relating to the Joint Venture or its business that may seriously affect the ability of the Joint Venture to perform its obligations under the joint venture agreement or may otherwise seriously impair the ability of the Joint Venture to conduct its business in an effective matter, other than one relating to the Joint Venture’s business plan or to factual matters that may be capable of expert determination; (ii) the effectiveness of the Company’s initial public offering, a change in control of the Company, the seventh anniversary of the formation of the Joint Venture, or each subsequent anniversary of each of the foregoing events; or (iii) a material breach of the joint venture agreement by the other party that goes unremedied within 20 business days. The purchase price per share of the Joint Venture will be equal to the average closing price of the shares for the 20 trading days ending on the business day immediately preceding the date of the put or call notice, if the shares of the Joint Venture are publicly traded and listed on a national exchange; or determined by a third-party valuation firm on the assumption that the sale is on an arm’s-length basis on the date of the put or call notice. Upon the effectiveness of the Company's IPO on October 3, 2018, the put-call rights for the Company to purchase all shares of the Joint Venture held by SoftBank are exercisable on each subsequent anniversary of the IPO by the Company or SoftBank. In the event the Company exercises its call right, the fair value of the Joint Venture will be deemed to be no less than an amount that yields a 20% internal rate of return on each tranche of capital invested by SoftBank and its affiliates in the Joint Venture, taking into account all proceeds received by SoftBank and its affiliates arising from their shares through such date. In the event SoftBank exercises its put right and the fair value of the Joint Venture is determined to be greater than 40% of the fair value of the Company, the Company will only be required to purchase the number of shares of the Joint Venture held by SoftBank and its affiliates having an aggregate value equal to the product of 40% and the pro rata portion of the outstanding shares of the Joint Venture held by SoftBank and its affiliates. The Company may pay the purchase price for the shares of the Joint Venture, in its sole discretion, in cash, in shares of its capital stock (which may be a non-voting security with senior preferences to all other classes of its equity or, if its common stock is publicly traded on a national exchange, its common stock), or in a combination thereof. The noncontrolling interest held by SoftBank contains embedded put-call redemption features that are not solely within the Company’s control and has been classified outside of permanent equity in the condensed consolidated balance sheets. The put-call feature embedded in the redeemable noncontrolling interest do not currently require bifurcation as it does not meet the definition of a derivative and is considered to be clearly and closely related to the redeemable noncontrolling interest. The noncontrolling interest is considered probable of becoming redeemable as SoftBank has the option to exercise its put right to sell its equity ownership in the Joint Venture to the Company on or after the seventh anniversary of the formation of the Joint Venture. The Company elected to recognize the change in redemption value immediately as they occur as if the put-call redemption feature were exercisable at the end of the reporting period. The carrying value of the redeemable noncontrolling interest is first adjusted for the earnings or losses attributable to the redeemable noncontrolling interest based on the percentage of the economic or ownership interest retained in the consolidated VIE by the noncontrolling parties, and then adjusted to equal to its redemption amount, or the fair value of the noncontrolling interest held by SoftBank, as if the redemption were to occur at the end of the reporting date. As of September 30, 2018, the fair value of the noncontrolling interest held by SoftBank approximates $42.0 million, and a fair value adjustment to redeemable noncontrolling interest of $950,000 was recorded in the Company’s condensed consolidated statements of operations during the three and nine months ended September 30, 2018.

Investment in Joint Venture

Variable Interest Entity (“VIE”)

In connection with SoftBank’s purchase of its Series E convertible preferred stock, the Company entered into a joint venture agreement with an entity affiliated with SoftBank, a related party. In May 2018, the Company and SoftBank formed and capitalized Guardant Health AMEA, Inc. (the “Joint Venture”) for the sale, marketing and distribution of the Company’s tests in all areas worldwide, outside of North America, Central America, South America, the United Kingdom, all other member states of the European Union as of May 2017, Iceland, Norway, Switzerland and Turkey. The Company expects to rely on the Joint Venture to accelerate commercialization of its products in Asia, the Middle East and Africa, with an initial focus on Japan.

Under the terms of the joint venture agreement, the Company paid $9.0 million for 40,000 shares of common stock, or 50% ownership interest, of the Joint Venture, and the affiliate of SoftBank contributed $41.0 million for 40,000 shares of common stock, or the other 50% ownership interest, of the Joint Venture. Neither party has the obligation to provide additional financial support to the Joint Venture. Each party holds two seats on the board of the Joint Venture and has to cast at least one vote for any board resolution of the Joint Venture to pass. The representatives of the Company on the Joint Venture’s board of directors have the right to appoint and remove a chief executive officer and a legal representative for the Joint Venture, in each case, subject to the approval of the full Joint Venture board of directors. The Joint Venture’s board of directors has the right to appoint and remove all other members of the Joint Venture’s senior management reporting to its chief executive officer and to approve the compensation of all foregoing individuals, including the compensation of the chief executive officer and legal representative.

At the inception of the arrangement and at the end of each reporting period, the Company assesses whether the Joint Venture is a variable interest entity (“VIE”), and if so, who is the primary beneficiary of the VIE. As of September 30, 2018, the Company and SoftBank had equal ownership interests and equal voting rights in the Joint Venture, and the Joint Venture’s board consisted of an equal number of directors representing the interest of the Company and SoftBank, respectively. As of September 30, 2018, the Joint Venture’s board had the right to vote on all critical matters that most significantly impact the Joint Venture’s economic performance, except that the Company had the unilateral right to make pricing decisions. As of September 30, 2018, the Company had responsibility for the Joint Venture’s daily operations, while SoftBank served as a financing partner. The Company also entered into various ancillary agreements with the Joint Venture necessary to operate its business. The Joint Venture is deemed to be a VIE and considering the power and benefits criterion, the Company and SoftBank, collectively as a related party group, has the characteristics of the primary beneficiary of the Joint Venture as the related party group has the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance and has the obligation to absorb losses or the right to receive benefits from the VIE that could potentially be significant to the VIE. Because the Company is most closely associated with the Joint Venture within the related party group, it has been identified as the VIE’s primary beneficiary. As the primary beneficiary, the Company has consolidated the financial position, results of operations and cash flows of the Joint Venture in its financial statements and all intercompany balances have been eliminated in consolidation. The Company concluded the Joint Venture did not meet the definition of a business upon consolidation as it lacked the processes required to generate outputs. Upon consolidation no liabilities were assumed and other than cash, and any identifiable assets were related to intellectual property rights that the Company transferred to the Joint Venture shortly before it became its primary beneficiary and therefore such transfer was treated as a common control transaction. Upon consolidation, the non-controlling interest of the affiliate of SoftBank was recorded at its estimated fair value of $41.0 million, which is equal to the original investment made by the affiliate of SoftBank.

As of September 30, 2018, the Joint Venture had total assets of approximately $49.5 million, which was primarily comprised of cash and security deposits. Although the Company consolidates the Joint Venture, the legal structure of the Joint Venture limits the recourse that its creditors will have over the Company’s general credit or assets. Similarly, the assets held in the Joint Venture can be used only to settle obligations of the Joint Venture. As of September 30, 2018, the Company has not provided financial or other support to the Joint Venture that was not previously contracted or required.

Put-call arrangements

The joint venture agreement includes a put-call arrangement with respect to the shares of the Joint Venture held by SoftBank and its affiliates. Under certain specified circumstances and on terms specified in the joint venture agreement, SoftBank will have the right to cause the Company to purchase all shares of the Joint Venture held by SoftBank and its affiliates, or the put right, and the Company will have a similar right to purchase all such shares, or the call right.

If the Company’s business model were to change such that the sale, marketing and distribution of its tests in the territory covered by the joint venture agreement was no longer economical, SoftBank would have the right to cause the Company to purchase, or the Company would have the right to purchase, all of the shares of the Joint Venture held by SoftBank and its affiliates. In this instance, the Company would be required to repurchase the shares at an aggregate purchase price of $41.0 million, the original purchase price paid by SoftBank to the Joint Venture for the shares.

Additionally, both the Company and SoftBank may exercise its respective put-call rights for the Company to purchase all shares of the Joint Venture held by SoftBank in the event of (i) certain material disagreement relating to the Joint Venture or its business that may seriously affect the ability of the Joint Venture to perform its obligations under the joint venture agreement or may otherwise seriously impair the ability of the Joint Venture to conduct its business in an effective matter, other than one relating to the Joint Venture’s business plan or to factual matters that may be capable of expert determination; (ii) the effectiveness of the Company’s initial public offering, a change in control of the Company, the seventh anniversary of the formation of the Joint Venture, or each subsequent anniversary of each of the foregoing events; or (iii) a material breach of the joint venture agreement by the other party that goes unremedied within 20 business days. The purchase price per share of the Joint Venture will be equal to the average closing price of the shares for the 20 trading days ending on the business day immediately preceding the date of the put or call notice, if the shares of the Joint Venture are publicly traded and listed on a national exchange; or determined by a third-party valuation firm on the assumption that the sale is on an arm’s-length basis on the date of the put or call notice. Upon the effectiveness of the Company's IPO on October 3, 2018, the put-call rights for the Company to purchase all shares of the Joint Venture held by SoftBank are exercisable on each subsequent anniversary of the IPO by the Company or SoftBank.

In the event the Company exercises its call right, the fair value of the Joint Venture will be deemed to be no less than an amount that yields a 20% internal rate of return on each tranche of capital invested by SoftBank and its affiliates in the Joint Venture, taking into account all proceeds received by SoftBank and its affiliates arising from their shares through such date.

In the event SoftBank exercises its put right and the fair value of the Joint Venture is determined to be greater than 40% of the fair value of the Company, the Company will only be required to purchase the number of shares of the Joint Venture held by SoftBank and its affiliates having an aggregate value equal to the product of 40% and the pro rata portion of the outstanding shares of the Joint Venture held by SoftBank and its affiliates.

The Company may pay the purchase price for the shares of the Joint Venture, in its sole discretion, in cash, in shares of its capital stock (which may be a non-voting security with senior preferences to all other classes of its equity or, if its common stock is publicly traded on a national exchange, its common stock), or in a combination thereof.

The noncontrolling interest held by SoftBank contains embedded put-call redemption features that are not solely within the Company’s control and has been classified outside of permanent equity in the condensed consolidated balance sheets. The put-call feature embedded in the redeemable noncontrolling interest do not currently require bifurcation as it does not meet the definition of a derivative and is considered to be clearly and closely related to the redeemable noncontrolling interest. The noncontrolling interest is considered probable of becoming redeemable as SoftBank has the option to exercise its put right to sell its equity ownership in the Joint Venture to the Company on or after the seventh anniversary of the formation of the Joint Venture. The Company elected to recognize the change in redemption value immediately as they occur as if the put-call redemption feature were exercisable at the end of the reporting period. The carrying value of the redeemable noncontrolling interest is first adjusted for the earnings or losses attributable to the redeemable noncontrolling interest based on the percentage of the economic or ownership interest retained in the consolidated VIE by the noncontrolling parties, and then adjusted to equal to its redemption amount, or the fair value of the noncontrolling interest held by SoftBank, as if the redemption were to occur at the end of the reporting date. As of September 30, 2018, the fair value of the noncontrolling interest held by SoftBank approximates $42.0 million, and a fair value adjustment to redeemable noncontrolling interest of $950,000 was recorded in the Company’s condensed consolidated statements of operations during the three and nine months ended September 30, 2018.