Entity Central Index Key Document And Entity Information [Abstract] Document And Entity Information [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well Known Seasoned Issuer Entity Current Reporting Status Entity Current Reporting Status Entity Voluntary Filers Entity Voluntary Filers Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Ex Transition Period Entity Ex Transition Period Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock Shares Outstanding Entity Public Float Entity Public Float Accrued liabilities and other liabilities current. Lease incentive payable noncurrent. Statement Of Financial Position [Abstract] Assets Assets [Abstract] Current assets: Assets Current [Abstract] Cash and cash equivalents Cash And Cash Equivalents At Carrying Value Investments Available For Sale Securities Debt Securities Current Prepaid expenses and other current assets Prepaid Expense And Other Assets Current Total current assets Assets Current Property and equipment, net Property Plant And Equipment Net Restricted investments Restricted Investments Noncurrent Restricted cash Restricted Cash Noncurrent Total assets Assets Liabilities and Stockholder's Equity (Deficit) Liabilities And Stockholders Equity [Abstract] Current liabilities: Liabilities Current [Abstract] Accounts payable Accounts Payable Current Accrued expenses and other current liabilities Accrued Liabilities And Other Liabilities Current Deferred revenue - related party Contract With Customer Liability Current Total current liabilities Liabilities Current Lease incentive obligation, net of current portion Lease Incentive Payable Noncurrent Deferred rent Deferred Rent Credit Noncurrent Deferred revenue, net of current portion - related party Contract With Customer Liability Noncurrent Other long-term liabilities Other Liabilities Noncurrent Total liabilities Liabilities Commitments and contingencies (Note 11) Commitments And Contingencies Stockholders’ equity (deficit): Stockholders Equity [Abstract] Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2018 and 2017; no shares issued and outstanding at December 31, 2018 and 2017 Preferred Stock Value Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2018 and 2017; 40,936,735 and 40,571,015 shares issued and outstanding at December 31, 2018 and 2017 Common Stock Value Additional paid-in capital Additional Paid In Capital Common Stock Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income Loss Net Of Tax Accumulated deficit Retained Earnings Accumulated Deficit Total stockholders’ equity (deficit) Stockholders Equity Total liabilities and stockholders’ equity (deficit) Liabilities And Stockholders Equity Preferred stock, par value Preferred Stock Par Or Stated Value Per Share Preferred stock, shares authorized Preferred Stock Shares Authorized Preferred stock, shares issued Preferred Stock Shares Issued Preferred stock, shares outstanding Preferred Stock Shares Outstanding Common stock, par value Common Stock Par Or Stated Value Per Share Common stock, shares authorized Common Stock Shares Authorized Common stock, shares issued Common Stock Shares Issued Common stock, shares outstanding Common Stock Shares Outstanding Revenue from Grant. Statement Of Income And Comprehensive Income [Abstract] Revenue: Revenues [Abstract] Collaboration revenue - related party Revenue From Contract With Customer Excluding Assessed Tax Type of Revenue [Extensible List] Type Of Revenue Extensible List Grant revenue Revenue From Grant Total revenue Revenues Operating expenses: Operating Expenses [Abstract] Research and development expenses Research And Development Expense General and administrative expenses General And Administrative Expense Total operating expenses Operating Expenses Loss from operations Operating Income Loss Other income (expense): Nonoperating Income Expense [Abstract] Interest income (expense), net Interest Income Expense Nonoperating Net Other income Other Nonoperating Income Total other income (expense), net Nonoperating Income Expense Net loss Net Income Loss Net loss per share attributable to common stockholders, basic and diluted Earnings Per Share Basic And Diluted Weighted average common shares outstanding, basic and diluted Weighted Average Number Of Share Outstanding Basic And Diluted Other comprehensive income (loss): Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract] Unrealized gain (loss) on investments, net of tax of $0 Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax Total other comprehensive income (loss) Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent Comprehensive loss Comprehensive Income Net Of Tax Unrealized (loss) gain on investment, tax Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax Stock repurchased during period share employee tax withholdings. Stock repurchased during period value employee tax withholdings. Statement Of Stockholders Equity [Abstract] Statement [Table] Statement [Table] Equity Components Statement Equity Components [Axis] Equity Component Equity Component [Domain] Common Stock [Member] Common Stock [Member] Additional Paid-in Capital [Member] Additional Paid In Capital [Member] Accumulated Other Comprehensive Income (Loss) [Member] Accumulated Other Comprehensive Income [Member] Accumulated Deficit [Member] Retained Earnings [Member] Statement [Line Items] Statement [Line Items] Beginning balance Beginning balance, shares Issuance of common stock upon exercise of stock options, value Stock Issued During Period Value Stock Options Exercised Issuance of common stock upon exercise of stock options, shares Stock Issued During Period Shares Stock Options Exercised Issuance of common stock upon vesting of RSUs, net of tax withholdings Restricted Stock Value Shares Issued Net Of Tax Withholdings Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares Restricted Stock Shares Issued Net Of Shares For Tax Withholdings Repurchase of common stock for employee tax withholdings Stock Repurchased During Period Value Employee Tax Withholdings Repurchase of common stock for employee tax withholdings, shares Stock Repurchased During Period Share Employee Tax Withholdings Issuance of common stock under ESPP plan, value Stock Issued During Period Value Employee Stock Purchase Plan Issuance of common stock under ESPP plan, shares Stock Issued During Period Shares Employee Stock Purchase Plans Stock-based compensation expense Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value Unrealized gain (loss) on investments Other Comprehensive Income Loss Available For Sale Securities Adjustment Before Reclassification Adjustments Net Of Tax Adoption of new revenue standard (ASC 606) New Accounting Pronouncement Or Change In Accounting Principle Effect Of Adoption Quantification Net loss Ending balance Ending balance, shares Noncash interest expense. Proceeds from issuance of common stock and restricted common stock. Statement Of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided By Used In Operating Activities [Abstract] Adjustments to reconcile net loss to net cash provided by (used in) operating activities: Adjustments To Reconcile Net Income Loss To Cash Provided By Used In Operating Activities [Abstract] Stock-based compensation expense Share Based Compensation Depreciation and amortization expense Depreciation Depletion And Amortization Non-cash interest expense Noncash Interest Expense Accretion of discount on investments Accretion Amortization Of Discounts And Premiums Investments Changes in operating assets and liabilities: Increase Decrease In Operating Capital [Abstract] Prepaid expenses and other current assets Increase Decrease In Prepaid Deferred Expense And Other Assets Deferred revenue Increase Decrease In Contract With Customer Liability Accounts payable Increase Decrease In Accounts Payable Accrued expenses and other liabilities Increase Decrease In Accrued Liabilities And Other Operating Liabilities Net cash provided by (used in) operating activities Net Cash Provided By Used In Operating Activities Cash flows from investing activities: Net Cash Provided By Used In Investing Activities [Abstract] Purchases of property and equipment Payments To Acquire Property Plant And Equipment Purchases of investments Payments To Acquire Investments Sales and maturities of investments Proceeds From Sale Maturity And Collections Of Investments Net cash provided by (used in) investing activities Net Cash Provided By Used In Investing Activities Cash flows from financing activities: Net Cash Provided By Used In Financing Activities [Abstract] Proceeds from exercise of stock options Proceeds From Stock Options Exercised Proceeds from issuance of common stock and restricted common stock Proceeds From Issuance Of Common Stock And Restricted Common Stock Payments for repurchase of common stock for employee tax withholdings Payments Related To Tax Withholding For Share Based Compensation Issuance of common stock under ESPP plan Proceeds From Stock Plans Net cash provided by financing activities Net Cash Provided By Used In Financing Activities Net increase (decrease) in cash, cash equivalents and restricted cash Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect Cash, cash equivalents and restricted cash at beginning of year Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Cash, cash equivalents and restricted cash at end of year Supplemental disclosure of cash flow information: Supplemental Cash Flow Information [Abstract] Cash paid for interest Interest Paid Net Supplemental disclosure of non-cash investing and financing activities: Noncash Investing And Financing Items [Abstract] Property and equipment purchases included in accounts payable and accrued expenses Capital Expenditures Incurred But Not Yet Paid Accounting Policies [Abstract] Nature of the Business and Basis of Presentation Business Description And Basis Of Presentation [Text Block] Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Fair Value Disclosures [Abstract] Fair Value of Financial Assets and Liabilities Fair Value Disclosures [Text Block] Investments Debt And Equity Securities [Abstract] Investments Investments In Debt And Marketable Equity Securities And Certain Trading Assets Disclosure [Text Block] Property Plant And Equipment [Abstract] Property and Equipment, Net Property Plant And Equipment Disclosure [Text Block] Payables And Accruals [Abstract] Accrued Expenses and Other Current Liabilities Accounts Payable Accrued Liabilities And Other Liabilities Disclosure Current [Text Block] Equity [Abstract] Convertible Preferred Stock Preferred Stock [Text Block] Stockholders' Equity Common Stock Shareholders Equity And Share Based Payments [Text Block] Revenue From Contract With Customer [Abstract] Collaboration Revenue Revenue From Contract With Customer [Text Block] Earnings Per Share [Abstract] Net Loss per Share Earnings Per Share [Text Block] Commitments And Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments And Contingencies Disclosure [Text Block] Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Quarterly Financial Data [Abstract] Selected Quarterly Financial Data (Unaudited) Quarterly Financial Information [Text Block] Related Party Transactions [Abstract] Related Party Transactions Related Party Transactions Disclosure [Text Block] Compensation And Retirement Disclosure [Abstract] 401(k) Savings Plan Pension And Other Postretirement Benefits Disclosure [Text Block] Subsequent Events [Abstract] Subsequent Events Subsequent Events [Text Block] Use of Estimates Use Of Estimates Cash Equivalents Cash And Cash Equivalents Policy [Text Block] Restricted Cash Cash And Cash Equivalents Restricted Cash And Cash Equivalents Policy Investments Investment Policy [Text Block] Restricted investments policy. Restricted Investments Restricted Investments Policy [Text Block] Concentration of Credit Risk and of Significant Suppliers Concentration Risk Credit Risk Fair Value Measurements Fair Value Measurement Policy Policy [Text Block] Property and Equipment Property Plant And Equipment Policy [Text Block] Impairment of Long-Lived Assets Impairment Or Disposal Of Long Lived Assets Policy [Text Block] Research and Development Costs Research And Development Expense Policy Research contract costs and accruals policy. Research Contract Costs and Accruals Research Contract Costs And Accruals Policy [Text Block] Patent costs policy. Patent Costs Patent Costs Policy [Text Block] Accounting for Stock-Based Compensation Share Based Compensation Option And Incentive Plans Policy Revenue Recognition Revenue Recognition Policy [Text Block] Collaborative Revenue Collaborative Arrangement Accounting Policy Grant revenue. Grant Revenue Grant Revenue Policy [Text Block] Income Taxes Income Tax Policy [Text Block] Segment Data Segment Reporting Policy Policy [Text Block] Comprehensive Loss Comprehensive Income Policy Policy [Text Block] Net Loss per Share Earnings Per Share Policy [Text Block] Recently Issued and Adopted Accounting Pronouncements New Accounting Pronouncements Policy Policy [Text Block] Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash Schedule Of Cash Cash Equivalents And Short Term Investments Table [Text Block] Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis Schedule Of Fair Value Assets And Liabilities Measured On Recurring Basis Table [Text Block] Schedule of Fair Value of Available-for-Sale Investments by Type of Security Debt Securities Available For Sale Table [Text Block] Schedule of Property and Equipment, Net Property Plant And Equipment [Text Block] Schedule of accrued expense and other current liabilities. Summary of Accrued Expenses and Other Current Liabilities Schedule Of Accrued Expense And Other Current Liabilities Table [Text Block] Summary of Fair Value of Stock Option Schedule Of Share Based Payment Award Stock Options Valuation Assumptions Table [Text Block] Summary of Stock Option Activity Disclosure Of Share Based Compensation Arrangements By Share Based Payment Award [Text Block] Summary of Restricted Stock Activity Schedule Of Sharebased Compensation Restricted Stock And Restricted Stock Units Activity Table [Text Block] Summary of Stock Based Compensation Expense Schedule Of Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Text Block] Adjustments to Consolidated Balance Sheet New Accounting Pronouncement Early Adoption Table [Text Block] Comparison of Reported Consolidated Balance Sheets, Statements of Operations and Comprehensive Loss and Cash Flows to Pro-forma Amounts Schedule Of Prospective Adoption Of New Accounting Pronouncements Table [Text Block] Changes in Contract Liabilities Contract With Customer Asset And Liability Table [Text Block]