Summary of Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2016
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed consolidated balance sheet as of September 30, 2016, the condensed consolidated statements of operations for the three and nine months ended September 30, 2016 and 2015, the condensed consolidated statements of cash flows for the nine months ended September 30, 2016 and 2015, and the condensed consolidated statements of convertible and redeemable convertible preferred stock and stockholders’ equity (deficit) as of September 30, 2016 are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s condensed consolidated financial statements included in this report. The condensed consolidated financial data disclosed in these notes to the condensed consolidated financial statements related to the three and nine month periods are also unaudited. The condensed consolidated results of operations for the three and nine months ended September 30, 2016 are not necessarily indicative of the results to be expected for the year ending December 31, 2016, or for any other future annual or interim period. The consolidated balance sheet as of December 31, 2015 included herein was derived from the audited consolidated financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015, filed with the SEC on March 3, 2016.
Use of Estimates	Use of Estimates The preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include, but are not limited to the fair value of stock options, accruals such as research and development costs, the fair value of common stock, the fair value of preferred stock, the fair value of preferred stock warrant liabilities, and recoverability of the Company’s net deferred tax assets and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates.
Foreign Currency	Foreign Currency The functional currency of the Company’s foreign subsidiaries is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are remeasured to the functional currency at the average exchange rate in effect during the period. At the end of each reporting period, monetary assets and liabilities are remeasured to the functional currency using exchange rates in effect at the balance sheet date. Non-monetary assets and liabilities are recorded at historical exchange rates. Gains and losses related to remeasurement are recorded in other income (expense) in the condensed consolidated statements of operations. The net foreign exchange transaction gains (losses) included in other income (expense) in the accompanying condensed consolidated statements of operations was insignificant for the three and nine months ended September 30, 2016 and 2015.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents consist primarily of funds invested in readily available checking and savings accounts and highly liquid investments in money market funds.
Restricted Cash	Restricted Cash Restricted cash represents collateral for a corporate credit card facility and security deposits required for facility leases. Restricted cash consists of funds invested in a money market fund. As of September 30, 2016, the current portion of restricted cash of $12,000 was included in prepaid expenses and other current assets and the long-term portion of restricted cash of $0.2 million was included in other assets in the accompanying condensed consolidated balance sheets. As of December 31, 2015, the current portion of restricted cash included in prepaid expenses and other current assets and the long-term portion of restricted cash included in other assets were $25,000 and $0.2 million, respectively.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that subject the Company to significant concentrations of credit risk consist of cash, cash equivalents and restricted cash. All of the Company’s cash, cash equivalents and restricted cash are held at financial institutions in the United States and Canada that management believes to be of high credit quality. Deposits held in the United States with these financial institutions exceed federally insured limits. The primary focus of the Company’s investment strategy is to preserve capital and meet liquidity requirements. The Company’s investment policy addresses the level of credit exposure by limiting the concentration in any one corporate issuer and establishing a minimum allowable credit rating.
Comprehensive Loss	Comprehensive Loss The Company had no components of comprehensive loss other than net loss for all periods presented.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, other current assets, accounts payable, and accrued liabilities approximate their fair value at September 30, 2016 and December 31, 2015, due to their short duration. The Company’s preferred stock warrant liabilities contain unobservable inputs that reflect the Company’s own assumptions in which there is little, if any, market activity at the measurement date; thus, the Company’s warrant liabilities were measured at fair value on a recurring basis using unobservable inputs until they were exercised in 2015. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 – Inputs are unadjusted quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 – Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 – Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net are stated at cost, less accumulated depreciation and amortization. Depreciation and amortization on property and equipment, excluding leasehold improvements, is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the condensed consolidated balance sheet and the resulting gain or loss is reflected in the condensed consolidated statement of operations. Leasehold improvements are amortized on a straight-line basis over the shorter of the estimated useful lives of the assets or the remaining lease term.
Other Assets	Other Assets Other assets consist primarily of restricted cash pledged as collateral for a corporate credit card facility and security deposits required for facility leases.
Preferred Stock Warrant Liabilities	Preferred Stock Warrant Liabilities The Company accounted for its warrants issued in connection with its various financing transactions based upon the characteristics and provisions of the instrument. Warrants classified as derivative liabilities were recorded on the Company’s condensed consolidated balance sheets at their fair value on the date of issuance and remeasured to fair value on each subsequent reporting period, with the changes in fair value recognized as a component of other income (expense), net in the accompanying condensed consolidated statements of operations. On the closing of the IPO on July 21, 2015 (discussed in Note 1), all outstanding warrants were exercised and the liability on the preferred stock warrants was reclassified to additional paid-in capital in stockholders’ equity (deficit), and was no longer subject to remeasurement.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. The Company accounts for non-refundable advance payments for goods and services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Depending on the timing of payments to service providers of research and development costs, the Company recognizes prepaid expenses or accrued expenses related to these costs. These prepaid or accrued expenses are based on management’s estimates of the work performed under service agreements and milestones achieved. In the event that a clinical trial is terminated early, the Company records an accrual for the estimated remaining costs to complete the trial in the period of termination. Upfront payments made in connection with license agreements are expensed as research and developments costs, as the assets acquired do not have alterative future use. Contingent milestone payment obligations due to third parties under license agreements are expensed when the milestones are considered probable of occurring. Research and development costs include fees incurred in connection with license agreements, compensation and other related costs for employees engaged in research and development, costs associated with preclinical studies and trials, regulatory activities, manufacturing activities to support clinical activities, license fees, fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company and an allocation of overhead expenses.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for share-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For share-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for employee stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis over the vesting period. For share-based awards that vest subject to the satisfaction of a service requirement and a performance component, the fair value measurement date is the date of grant and is recognized over the requisite service period as achievement of the performance objective becomes probable. Stock-based compensation arrangements with non-employees are recognized at the grant date and remeasured to fair value at each reporting period. The expense is recognized over the vesting period, which is generally the service period.
Income Taxes	Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net U.S. deferred tax assets have been offset by a full valuation allowance. The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company recognizes interest and penalties related to the underpayment of income taxes as a component of provision for income taxes.
Segment Information	Segment Information Operating segments are components of an enterprise for which separate financial information is available and is evaluated regularly by the Company’s chief operating decision maker in deciding how to allocate resources and assessing performance. The Company’s chief operating decision maker is its Chief Executive Officer. The Company’s Chief Executive Officer views the Company’s operations and manages its business in one operating segment, which is the business of researching, developing and commercializing therapies for the treatment of patients with cancer. Accordingly, the Company has a single reporting segment.
Recent Accounting Pronouncements Not Yet Effective	Recent Accounting Pronouncements Not Yet Effective In February 2016, the Financial Accounting Standards Board (FASB) issued FASB Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842). The amendments in this update require that organizations recognize lease assets and lease liabilities on the balance sheet and disclose key information about leasing arrangements. This ASU is effective for financial statements issued for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, with early adoption permitted. The Company is currently assessing the impact of the ASU on the Company’s consolidated financial statements. In March 2016, the FASB issued FASB ASU No. 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. The amendments in this update involve the simplification of several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This ASU is effective for financial statements issued for annual periods beginning after December 15, 2016, and interim periods within those annual periods, with early adoption permitted. The Company is currently assessing the impact of the ASU on the Company’s consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2016

Basis of Presentation

The condensed consolidated balance sheet as of September 30, 2016, the condensed consolidated statements of operations for the three and nine months ended September 30, 2016 and 2015, the condensed consolidated statements of cash flows for the nine months ended September 30, 2016 and 2015, and the condensed consolidated statements of convertible and redeemable convertible preferred stock and stockholders’ equity (deficit) as of September 30, 2016 are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s condensed consolidated financial statements included in this report. The condensed consolidated financial data disclosed in these notes to the condensed consolidated financial statements related to the three and nine month periods are also unaudited. The condensed consolidated results of operations for the three and nine months ended September 30, 2016 are not necessarily indicative of the results to be expected for the year ending December 31, 2016, or for any other future annual or interim period. The consolidated balance sheet as of December 31, 2015 included herein was derived from the audited consolidated financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015, filed with the SEC on March 3, 2016.

Use of Estimates

Use of Estimates

The preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include, but are not limited to the fair value of stock options, accruals such as research and development costs, the fair value of common stock, the fair value of preferred stock, the fair value of preferred stock warrant liabilities, and recoverability of the Company’s net deferred tax assets and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates.

Foreign Currency

Foreign Currency

The functional currency of the Company’s foreign subsidiaries is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are remeasured to the functional currency at the average exchange rate in effect during the period. At the end of each reporting period, monetary assets and liabilities are remeasured to the functional currency using exchange rates in effect at the balance sheet date. Non-monetary assets and liabilities are recorded at historical exchange rates. Gains and losses related to remeasurement are recorded in other income (expense) in the condensed consolidated statements of operations. The net foreign exchange transaction gains (losses) included in other income (expense) in the accompanying condensed consolidated statements of operations was insignificant for the three and nine months ended September 30, 2016 and 2015.

Cash and Cash Equivalents

Cash and Cash Equivalents

The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents consist primarily of funds invested in readily available checking and savings accounts and highly liquid investments in money market funds.

Restricted Cash

Restricted Cash

Restricted cash represents collateral for a corporate credit card facility and security deposits required for facility leases. Restricted cash consists of funds invested in a money market fund. As of September 30, 2016, the current portion of restricted cash of $12,000 was included in prepaid expenses and other current assets and the long-term portion of restricted cash of $0.2 million was included in other assets in the accompanying condensed consolidated balance sheets. As of December 31, 2015, the current portion of restricted cash included in prepaid expenses and other current assets and the long-term portion of restricted cash included in other assets were $25,000 and $0.2 million, respectively.

Concentrations of Credit Risk

Concentrations of Credit Risk

Financial instruments that subject the Company to significant concentrations of credit risk consist of cash, cash equivalents and restricted cash. All of the Company’s cash, cash equivalents and restricted cash are held at financial institutions in the United States and Canada that management believes to be of high credit quality. Deposits held in the United States with these financial institutions exceed federally insured limits.

The primary focus of the Company’s investment strategy is to preserve capital and meet liquidity requirements. The Company’s investment policy addresses the level of credit exposure by limiting the concentration in any one corporate issuer and establishing a minimum allowable credit rating.

Comprehensive Loss

Comprehensive Loss

The Company had no components of comprehensive loss other than net loss for all periods presented.

Fair Value of Financial Instruments

Fair Value of Financial Instruments

The Company’s cash and cash equivalents, restricted cash, other current assets, accounts payable, and accrued liabilities approximate their fair value at September 30, 2016 and December 31, 2015, due to their short duration. The Company’s preferred stock warrant liabilities contain unobservable inputs that reflect the Company’s own assumptions in which there is little, if any, market activity at the measurement date; thus, the Company’s warrant liabilities were measured at fair value on a recurring basis using unobservable inputs until they were exercised in 2015.

The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels:

Level 1 – Inputs are unadjusted quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 2 – Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and

Level 3 – Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.

Property and Equipment, Net

Property and Equipment, Net

Property and equipment, net are stated at cost, less accumulated depreciation and amortization. Depreciation and amortization on property and equipment, excluding leasehold improvements, is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the condensed consolidated balance sheet and the resulting gain or loss is reflected in the condensed consolidated statement of operations. Leasehold improvements are amortized on a straight-line basis over the shorter of the estimated useful lives of the assets or the remaining lease term.

Other Assets

Other Assets

Other assets consist primarily of restricted cash pledged as collateral for a corporate credit card facility and security deposits required for facility leases.

Preferred Stock Warrant Liabilities

Preferred Stock Warrant Liabilities

The Company accounted for its warrants issued in connection with its various financing transactions based upon the characteristics and provisions of the instrument. Warrants classified as derivative liabilities were recorded on the Company’s condensed consolidated balance sheets at their fair value on the date of issuance and remeasured to fair value on each subsequent reporting period, with the changes in fair value recognized as a component of other income (expense), net in the accompanying condensed consolidated statements of operations. On the closing of the IPO on July 21, 2015 (discussed in Note 1), all outstanding warrants were exercised and the liability on the preferred stock warrants was reclassified to additional paid-in capital in stockholders’ equity (deficit), and was no longer subject to remeasurement.

Research and Development Costs

Research and Development Costs

Research and development costs are expensed as incurred. The Company accounts for non-refundable advance payments for goods and services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Depending on the timing of payments to service providers of research and development costs, the Company recognizes prepaid expenses or accrued expenses related to these costs. These prepaid or accrued expenses are based on management’s estimates of the work performed under service agreements and milestones achieved. In the event that a clinical trial is terminated early, the Company records an accrual for the estimated remaining costs to complete the trial in the period of termination.

Upfront payments made in connection with license agreements are expensed as research and developments costs, as the assets acquired do not have alterative future use. Contingent milestone payment obligations due to third parties under license agreements are expensed when the milestones are considered probable of occurring.

Research and development costs include fees incurred in connection with license agreements, compensation and other related costs for employees engaged in research and development, costs associated with preclinical studies and trials, regulatory activities, manufacturing activities to support clinical activities, license fees, fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company and an allocation of overhead expenses.

Stock-Based Compensation

Stock-Based Compensation

The Company accounts for share-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For share-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for employee stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis over the vesting period.

For share-based awards that vest subject to the satisfaction of a service requirement and a performance component, the fair value measurement date is the date of grant and is recognized over the requisite service period as achievement of the performance objective becomes probable.

Stock-based compensation arrangements with non-employees are recognized at the grant date and remeasured to fair value at each reporting period. The expense is recognized over the vesting period, which is generally the service period.

Income Taxes

Income Taxes

The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net U.S. deferred tax assets have been offset by a full valuation allowance.

The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company recognizes interest and penalties related to the underpayment of income taxes as a component of provision for income taxes.

Segment Information

Segment Information

Operating segments are components of an enterprise for which separate financial information is available and is evaluated regularly by the Company’s chief operating decision maker in deciding how to allocate resources and assessing performance. The Company’s chief operating decision maker is its Chief Executive Officer.

The Company’s Chief Executive Officer views the Company’s operations and manages its business in one operating segment, which is the business of researching, developing and commercializing therapies for the treatment of patients with cancer. Accordingly, the Company has a single reporting segment.

Recent Accounting Pronouncements Not Yet Effective

Recent Accounting Pronouncements Not Yet Effective

In February 2016, the Financial Accounting Standards Board (FASB) issued FASB Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842). The amendments in this update require that organizations recognize lease assets and lease liabilities on the balance sheet and disclose key information about leasing arrangements. This ASU is effective for financial statements issued for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, with early adoption permitted. The Company is currently assessing the impact of the ASU on the Company’s consolidated financial statements.

In March 2016, the FASB issued FASB ASU No. 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. The amendments in this update involve the simplification of several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This ASU is effective for financial statements issued for annual periods beginning after December 15, 2016, and interim periods within those annual periods, with early adoption permitted. The Company is currently assessing the impact of the ASU on the Company’s consolidated financial statements.