Investment in Joint Venture	6 Months Ended
Jun. 30, 2019
Equity Method Investments and Joint Ventures [Abstract]
Investment in Joint Venture	Investment in Joint Venture Variable Interest Entity (“VIE”) 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. The Joint Venture is deemed to be a variable interest entity (“VIE”) and the Company 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. As of June 30, 2019 and December 31, 2018, the Joint Venture had total assets of approximately $45.1 million and $48.3 million, respectively, which were primarily comprised of cash, property and equipment 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 June 30, 2019 and December 31, 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, including timely written notice, SoftBank has the right to cause the Company to purchase all shares of the Joint Venture held by SoftBank and its affiliates (the “put right”), and the Company has a right to purchase all such shares (the “call right”). Each of 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 in these situations 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. As a result of the IPO, 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% of the Company's fair value 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 cash, in shares of its common stock, or in a combination thereof. In the event the Company exercises the call right, SoftBank will choose the form of consideration. In the event SoftBank exercises the put right, the Company will choose the form of consideration. 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 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, on each subsequent anniversary of the IPO and under certain other circumstances. 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. As of June 30, 2019 and December 31, 2018, the fair value of the redeemable noncontrolling interest held by SoftBank approximated $46.8 million and $41.8 million, respectively. For the three and six months ended June 30, 2019, the Company recorded a fair value adjustment of $0.3 million and $5.0 million , respectively, in its condensed consolidated statements of operations. As of June 30, 2019, the fair value of the redeemable noncontrolling interest held by SoftBank was determined using the combination of the income approach and the market approach. Determining the fair value of the redeemable noncontrolling interest requires judgment and the use of significant estimates and assumptions. Such estimates and assumptions include future revenue growth rates, gross profit margins, EBITDA margins, future capital expenditures, weighted average costs of capital and future market conditions, among others. The fair value measurement of the redeemable noncontrolling interest is classified within Level 3 of the fair value hierarchy.

Investment in Joint Venture

Variable Interest Entity (“VIE”)

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. The Joint Venture is deemed to be a variable interest entity (“VIE”) and the Company 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.

As of June 30, 2019 and December 31, 2018, the Joint Venture had total assets of approximately $45.1 million and $48.3 million, respectively, which were primarily comprised of cash, property and equipment 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 June 30, 2019 and December 31, 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, including timely written notice, SoftBank has the right to cause the Company to purchase all shares of the Joint Venture held by SoftBank and its affiliates (the “put right”), and the Company has a right to purchase all such shares (the “call right”).

Each of 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 in these situations 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. As a result of the IPO, 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% of the Company's fair value 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 cash, in shares of its common stock, or in a combination thereof. In the event the Company exercises the call right, SoftBank will choose the form of consideration. In the event SoftBank exercises the put right, the Company will choose the form of consideration.

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 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, on each subsequent anniversary of the IPO and under certain other circumstances. 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.

As of June 30, 2019 and December 31, 2018, the fair value of the redeemable noncontrolling interest held by SoftBank approximated $46.8 million and $41.8 million, respectively. For the three and six months ended June 30, 2019, the Company recorded a fair value adjustment of $0.3 million and $5.0 million , respectively, in its condensed consolidated statements of operations.

As of June 30, 2019, the fair value of the redeemable noncontrolling interest held by SoftBank was determined using the combination of the income approach and the market approach. Determining the fair value of the redeemable noncontrolling interest requires judgment and the use of significant estimates and assumptions. Such estimates and assumptions include future revenue growth rates, gross profit margins, EBITDA margins, future capital expenditures, weighted average costs of capital and future market conditions, among others. The fair value measurement of the redeemable noncontrolling interest is classified within Level 3 of the fair value hierarchy.