Revenue Recognition [Abstract] Long-term Purchase Commitment [Table] Long-term Purchase Commitment [Table] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Preferred Stock Preferred Stock [Member] Common stock of U.S. corporation Common Stock [Member] Type of Arrangement and Non-arrangement Transactions [Axis] Type of Arrangement and Non-arrangement Transactions [Axis] Arrangements and Non-arrangement Transactions [Domain] Arrangements and Non-arrangement Transactions [Domain] The Regents of University of California Regents Of University Of California [Member] Regents Of University Of California [Member]. BARDA BARDA [Member] BARDA [Member] BARDA Second Option Segment BARDA Second Option Segment [Member] BARDA Second Option Segment [Member] BARDA Third Option Segment BARDA Third Option Segment [Member] BARDA Third Option Segment [Member] ContraVir Pharmaceuticals ContraVir Pharmaceuticals [Member] ContraVir Pharmaceuticals [Member] The Regents of the University of Michigan The Regents of the University of Michigan [Member] The Regents of the University of Michigan Range [Axis] Range [Axis] Range [Domain] Range [Domain] Maximum Maximum [Member] Minimum Minimum [Member] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] Brincidofovir Brincidofovir [Member] Brincidofovir [Member]. Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Scenario, Forecast Scenario, Forecast [Member] Valuation Technique [Axis] Valuation Technique [Axis] Valuation Technique [Domain] Valuation Technique [Domain] Stated Value Stated Value [Member] Stated Value [Member] Long-term Purchase Commitment [Line Items] Long-term Purchase Commitment [Line Items] Milestone payments Milestone Payments Aggregate Payments for certain milestones achieved. Number of options outstanding, granted (in shares) Stock Issued During Period, Shares, Other Sublicense fee percentage Sublicense Fee Percentage Sublicense fee percentage. Sublicense fee amount, payable fee percentage Sublicense Fee Amount Payable Fee Percentage Sublicense fee amount, payable fee percentage. Royalties percentage Royalties Percentage Royalties percentage. Operating expenses Operating Expenses Contract revenue Contracts Revenue Number of extension periods Number Of Extension Periods The number of extension periods. Reimbursement revenue Reimbursement Revenue Fees and commissions, other Fees and Commissions, Other Other significant noncash transaction, shares received as consideration (in shares) Other Significant Noncash Transaction, Shares Received as Consideration Other Significant Noncash Transaction, Shares Received as Consideration Other significant noncash transaction, value of consideration received Other Significant Noncash Transaction, Value of Consideration Received Collaboration and licensing revenue Future Licenses Revenue Future Licenses Revenue License agreement, termination notice License Agreement, Termination Notice License Agreement, Termination Notice Estimated Fair Value Available-for-sale Securities Licenses revenues Licenses Revenue Convertible preferred stock, shares issued upon conversion (in shares) Convertible Preferred Stock, Shares Issued upon Conversion Cost of services, licenses and services Cost of Services, Licenses and Services Stock issued during period, shares, issued for services Stock Issued During Period, Shares, Issued for Services Investments, Debt and Equity Securities [Abstract] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Maturing in one year or less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Maturing after one year through two years Available-for-sale Securities, Debt Maturities, Year Two, Fair Value Available-for-sale Securities, Debt Maturities, Year Two, Fair Value Total debt investments Available-for-sale Securities, Debt Securities Estimated fair value Summary of Short-term and Long-term investments Available-for-sale Securities [Table Text Block] Summary of Investments with Unrealized Losses, Aggregated by Investment Type and the Length of Time Schedule of Unrealized Loss on Investments [Table Text Block] Summary of the scheduled maturity of Company investments Investments Classified by Contractual Maturity Date [Table Text Block] 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 Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Income Tax Disclosure [Abstract] Income Tax Contingency [Table] Income Tax Contingency [Table] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Estimated annual effective tax rate Effective Income Tax Rate Reconciliation, Percent Federal or state income taxes Income Tax Expense (Benefit) Unrecognized tax benefits Unrecognized Tax Benefits Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Equity Transactions and Share-based Compensation Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Schedule of Share-Based Compensation Expense Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] Income Taxes Income Tax Disclosure [Text Block] Subsequent Events [Abstract] Subsequent Events Subsequent Events [Text Block] Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Current assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Short-term investments, available-for-sale Available-for-sale Securities, Current Accounts receivable Accounts Receivable, Net, Current Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Total current assets Assets, Current Long-term investments Available-for-sale Securities, Noncurrent Property and equipment, net of accumulated depreciation Property, Plant and Equipment, Net Other long-term assets Other Assets, Noncurrent Total assets Assets LIABILITIES AND STOCKHOLDERS' EQUITY Liabilities and Equity [Abstract] Current liabilities: Liabilities, Current [Abstract] Accounts payable Accounts Payable, Current Accrued liabilities Accrued Liabilities, Current Total current liabilities Liabilities, Current Lease-related obligations Deferred Rent Credit, Noncurrent Total liabilities Liabilities Stockholders’ equity: Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Preferred stock, $0.001 par value, 10,000,000 shares authorized at March 31, 2017 and December 31, 2016; no shares issued and outstanding as of March 31, 2017 and December 31, 2016 Preferred Stock, Value, Outstanding Common stock, $0.001 par value, 200,000,000 shares authorized at March 31, 2017 and December 31, 2016; 46,651,793 and 46,522,475 shares issued and outstanding as of March 31, 2017 and December 31, 2016, respectively Common Stock, Value, Outstanding Additional paid-in capital Additional Paid in Capital Accumulated other comprehensive loss, net Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated deficit Retained Earnings (Accumulated Deficit) Total stockholders’ equity Stockholders' Equity Attributable to Parent Total liabilities and stockholders’ equity Liabilities and Equity Significant Agreements Research and Development Arrangement, Contract to Perform for Others [Table Text Block] Other Liabilities Disclosure [Abstract] Accrued research and development expenses Accrued Development Liabilities Accrued development liabilities. Accrued compensation Accrued Salaries Other accrued liabilities Other Accrued Liabilities, Current Total accrued liabilities Income Statement [Abstract] Operating expenses: Operating Expenses [Abstract] Research and development Research and Development Expense General and administrative General and Administrative Expense Total operating expenses Loss from operations Operating Income (Loss) Interest income Interest Income (Expense), Net Net loss Net Income (Loss) Attributable to Parent Other comprehensive loss: Other Comprehensive Income (Loss), Tax [Abstract] Unrealized gain on investments, net Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax Comprehensive loss Comprehensive Income (Loss), Net of Tax, Attributable to Parent Per share information: Earnings Per Share [Abstract] Net loss, basic and diluted (in dollars per share) Earnings Per Share, Basic and Diluted Weighted-average shares outstanding, basic and diluted (in shares) Weighted Average Number of Shares Outstanding, Basic and Diluted Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Employee Employees [Member] Employees [Member] Non-employee Non-employee [Member] Non-employee Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Research and development expense Research and Development Expense [Member] General and administrative expense General and Administrative Expense [Member] Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] Allocated share-based compensation expense Allocated Share-based Compensation Expense Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Preferred stock shares issued (in shares) Preferred Stock, Shares Issued Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common stock, shares authorized (in shares) Common Stock, Shares Authorized Common stock, shares issued (in shares) Common Stock, Shares, Issued Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Accounting Policies [Abstract] Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Fair Value of Financial Instruments Fair Value of Financial Instruments, Policy [Policy Text Block] Revenue Recognition Revenue Recognition, Policy [Policy Text Block] Research and Development Prepaids and Accruals Research and Development Expense, Policy [Policy Text Block] Basic and Diluted Net Loss Per Share of Common Stock Earnings Per Share, Policy [Policy Text Block] Impact of Recently Issued Accounting Standards and Impact of Recently Adopted Accounting Standards New Accounting Pronouncements, Policy [Policy Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Performance Shares Performance Shares [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] The 2013 Plan The 2013 Plan [Member] The 2013 Plan [Member] The 2013 Employee Stock Purchase Plan The 2013 Employee Stock Purchase Plan [Member] The 2013 Employee Stock Purchase Plan Member. Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based compensation, evergreen option provision, equity increase Share-Based Compensation, Evergreen Option Provision, Equity Increase Share-Based Compensation, Evergreen Option Provision, Equity Increase Additional shares authorized (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized Share-based compensation arrangement by share-based payment award, number of shares available for grant (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Share-based compensation arrangement by share-based payment award, options, exercises in period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Number of shares authorized to be granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Number of shares reserved for issuance, annual increase (in shares) Increase (Decrease) in Number of Shares Authorized Increase (decrease) in number of shares authorized. Percentage of pay that employee can contribute, maximum Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate Employee stock purchase plan, automatic reset purchase period Employee Stock Purchase Plan, Automatic Reset Purchase Period Employee Stock Purchase Plan, Automatic Reset Purchase Period Employee stock purchase plan, purchase period Employee Stock Purchase Plan, Purchase Period Employee Stock Purchase Plan, Purchase Period Discounted purchase price from market price, offering date Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Offering Date Discounted purchase price from market price, purchase date Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date Stock issued during period, shares, restricted stock award, gross (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Gross Assets and Liabilities Measured at Fair Value on a Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Schedule of Accrued Liabilities Schedule of Accrued Liabilities [Table Text Block] Schedule of Fair Value of Separate Accounts by Major Category of Investment [Table] Schedule of Fair Value of Separate Accounts by Major Category of Investment [Table] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] Anti-dilutive securities excluded from the calculation loss per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount The Business and Summary of Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Money market funds Money Market Funds [Member] Commercial paper Commercial Paper [Member] Certificates of deposit Certificates of Deposit [Member] U.S. treasury securities US Treasury Securities [Member] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair value, measurements, recurring Fair Value, Measurements, Recurring [Member] Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Short-term investments Long-term investments Total assets Assets, Fair Value Disclosure Fair Value, Less than 12 Months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value Unrealized Loss, Less than 12 Months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Fair Value, Greater than 12 Months Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Unrealized Loss, Greater than 12 Months Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Fair Value, Total Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Unrealized Loss, Total Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss Number of securities with unrealized losses, Less than 12 Months Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year Number of securities with unrealized losses, Greater than 12 Months Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year Number of securities with unrealized losses, Total Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions Other than temporary impairment, investments, available-for-sale Other than Temporary Impairment Losses, Investments, Available-for-sale Securities Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Class of Stock [Line Items] Class of Stock [Line Items] Shares issued for warrants exercised (in shares) Shares Issued for Warrants Exercised Shares issued for warrants exercised Stock issued during period, shares, employee stock purchase plans (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans Schedule of Investments [Table] Schedule of Investments [Table] Schedule of Investments [Line Items] Schedule of Investments [Line Items] Amortized Cost Available-for-sale Securities, Amortized Cost Basis Gross Unrealized Gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Operating leases, rent expense Operating Leases, Rent Expense, Net Other Commitments [Table] Other Commitments [Table] Other Commitments [Line Items] Other Commitments [Line Items] Deferred revenue Deferred Revenue Statement of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net loss Adjustments to reconcile net loss to net cash used in operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation of property and equipment Depreciation Amortization of premium/discount on investments Accretion (Amortization) of Discounts and Premiums, Investments Share-based compensation Share-based Compensation Amortization of lease-related obligations Increase (Decrease) in Deferred Liabilities Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Accounts receivable Increase (Decrease) in Accounts Receivable Prepaid expenses and other assets Increase (Decrease) in Prepaid Expense and Other Assets Accounts payable and accrued liabilities Increase (Decrease) in Accounts Payable and Accrued Liabilities Net cash 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 long-term investments Payments to Acquire Long-term Investments Proceeds from maturities of short-term investments Proceeds from Maturities, Prepayments and Calls of Short-term Investments Net cash (used in) provided by 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 employee stock purchase plan Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Excluding Stock Options Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Net (decrease) increase in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents: Cash and Cash Equivalents [Abstract] Beginning of period End of period Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block]