Description of Business	9 Months Ended
Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business Guardant Health, Inc. (the “Company”) is a leading precision oncology company focused on helping conquer cancer globally through use of its proprietary blood tests, vast data sets and advanced analytics. The key to conquering cancer is unprecedented access to its molecular information throughout all stages of the disease, which it enables by a routine blood draw, or liquid biopsy. The Guardant Health Oncology Platform is designed to leverage the Company’s capabilities in technology, clinical development, regulatory, reimbursement and commercial adoption to improve patient clinical outcomes, lower healthcare costs and accelerate biopharmaceutical drug development. In pursuit of its goal to manage cancer across all stages of the disease, it has launched multiple liquid biopsy-based tests, Guardant360 and GuardantOMNI for advanced stage cancer patients, which fuel its development programs for recurrence and early detection, LUNAR-1 and LUNAR-2, respectively. Guardant360, which the Company launched in 2014, has been used by oncologists, biopharmaceutical companies and National Comprehensive Cancer Network cancer centers. GuardantOMNI, a purpose-built comprehensive genomic profiling tool to enable the Company’s biopharmaceutical customers to accelerate clinical development programs in both the immuno-oncology and targeted therapy areas, was launched in 2017. The Company was incorporated in Delaware in December 2011 and is headquartered in Redwood City, California. In April 2018, the Company established Guardant Health AMEA, Inc. (the “Joint Venture”) in the United States with an entity affiliated with SoftBank. Under the terms of the joint venture agreement, the Company held a 50% ownership interest in the Joint Venture. As of September 30, 2018, the Joint Venture has branch offices in Singapore and Japan (see Note 3). Approval of Amended and Restated Certificate of Incorporation In September 2018, the Company’s Board of Directors and its stockholders approved a 0.7378-for-one reverse stock split of the Company’s common stock. The reverse stock split became effective on September 19, 2018. The par value of the common stock was not adjusted as a result of the reverse stock split. Adjustments corresponding to the reverse stock split were made to the ratio at which the convertible preferred stock was convertible into common stock immediately prior to the closing of the IPO. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse split. Initial Public Offering On October 3, 2018, the Company completed its initial public offering (the “IPO”) in which it issued and sold 14,375,000 shares of its common stock at a public offering price of $19.00 per share. The Company received net proceeds of $249.5 million after deducting underwriting discounts and commissions and offering expenses. All outstanding common stock warrants were exercised prior to the completion of the IPO. In addition, in connection with the IPO, all shares of the Company’s then-outstanding convertible preferred stock were automatically converted into 58,264,577 shares of its common stock at their respective conversion ratios, and all shares of the Company's then-outstanding warrants to purchase preferred stock were automatically converted into warrants to purchase 7,636 shares of its common stock.

Description of Business

Guardant Health, Inc. (the “Company”) is a leading precision oncology company focused on helping conquer cancer globally through use of its proprietary blood tests, vast data sets and advanced analytics. The key to conquering cancer is unprecedented access to its molecular information throughout all stages of the disease, which it enables by a routine blood draw, or liquid biopsy. The Guardant Health Oncology Platform is designed to leverage the Company’s capabilities in technology, clinical development, regulatory, reimbursement and commercial adoption to improve patient clinical outcomes, lower healthcare costs and accelerate biopharmaceutical drug development. In pursuit of its goal to manage cancer across all stages of the disease, it has launched multiple liquid biopsy-based tests, Guardant360 and GuardantOMNI for advanced stage cancer patients, which fuel its development programs for recurrence and early detection, LUNAR-1 and LUNAR-2, respectively. Guardant360, which the Company launched in 2014, has been used by oncologists, biopharmaceutical companies and National Comprehensive Cancer Network cancer centers. GuardantOMNI, a purpose-built comprehensive genomic profiling tool to enable the Company’s biopharmaceutical customers to accelerate clinical development programs in both the immuno-oncology and targeted therapy areas, was launched in 2017.

The Company was incorporated in Delaware in December 2011 and is headquartered in Redwood City, California. In April 2018, the Company established Guardant Health AMEA, Inc. (the “Joint Venture”) in the United States with an entity affiliated with SoftBank. Under the terms of the joint venture agreement, the Company held a 50% ownership interest in the Joint Venture. As of September 30, 2018, the Joint Venture has branch offices in Singapore and Japan (see Note 3).

Approval of Amended and Restated Certificate of Incorporation

In September 2018, the Company’s Board of Directors and its stockholders approved a 0.7378-for-one reverse stock split of the Company’s common stock. The reverse stock split became effective on September 19, 2018. The par value of the common stock was not adjusted as a result of the reverse stock split. Adjustments corresponding to the reverse stock split were made to the ratio at which the convertible preferred stock was convertible into common stock immediately prior to the closing of the IPO. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse split.

Initial Public Offering

On October 3, 2018, the Company completed its initial public offering (the “IPO”) in which it issued and sold 14,375,000 shares of its common stock at a public offering price of $19.00 per share. The Company received net proceeds of $249.5 million after deducting underwriting discounts and commissions and offering expenses. All outstanding common stock warrants were exercised prior to the completion of the IPO. In addition, in connection with the IPO, all shares of the Company’s then-outstanding convertible preferred stock were automatically converted into 58,264,577 shares of its common stock at their respective conversion ratios, and all shares of the Company's then-outstanding warrants to purchase preferred stock were automatically converted into warrants to purchase 7,636 shares of its common stock.