Document and Entity Information Document Information [Table] Document Information [Line Items] Document Type Document Period End Date Entity Registrant Name Entity Current Reporting Status Entity Interactive Data Current Entity Filer Category Entity Small Business Entity Emerging Growth Company Entity Ex Transition Period Entity Shell Company Entity Common Stock, Shares Outstanding Entity Central Index Key Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Period Focus Amendment Flag Condensed Consolidated Balance Sheets Statement [Table] Statement [Line Items] Assets [Abstract] Assets Assets, Current [Abstract] Current assets: Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents Prepaid Expense and Other Assets, Current Prepaid expenses and other current assets Assets, Current Total current assets Property, Plant and Equipment, Net Property and equipment, net Operating Lease, Right-of-Use Asset Right of use lease asset Other Assets, Noncurrent Other noncurrent assets Assets Total assets Liabilities and Equity [Abstract] Liabilities, Convertible Preferred Stock and Stockholders' Deficit Liabilities, Current [Abstract] Current liabilities: Accounts Payable, Current Accounts payable The carrying values as of the balance sheet date of obligations incurred through that date and payable for obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Also includes obligations related to services received from employees, such as accrued salaries and bonuses, payroll taxes and fringe benefits. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). Accrued Liabilities and Employee-related Liabilities, Current Accrued expenses Total accrued expenses Operating Lease, Liability, Current Lease liability—current Liabilities, Current Total current liabilities Operating Lease, Liability, Noncurrent Lease liability—noncurrent Liabilities Total liabilities Commitments and Contingencies Commitments and contingencies (Note 7) Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Stockholders’ equity: Common Stock, Value, Issued Common stock, $0.001 par value, 400,000,000 shares authorized, 21,054,872 and 21,022,752 shares issued and outstanding at March 31, 2020 and December 31, 2019 , respectively. Additional Paid in Capital Additional paid-in capital Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated other comprehensive loss Retained Earnings (Accumulated Deficit) Accumulated deficit Stockholders' Equity Attributable to Parent Equity, ending balance Equity, beginning balance Total stockholders’ equity Liabilities and Equity Total liabilities and stockholders’ equity Common Stock, Par or Stated Value Per Share Common stock, par value Common Stock, Shares Authorized Common stock, authorized Common Stock, Shares, Issued Common stock, shares issued Common Stock, Shares, Outstanding Common stock, outstanding Condensed Consolidated Statements of Operations and Comprehensive Loss Operating Expenses [Abstract] Operating expenses: Research and Development Expense Research and development General and Administrative Expense General and administrative Operating Expenses Total operating expenses Nonoperating Income (Expense) [Abstract] Other income (expense): Interest Income (Expense), Nonoperating, Net Interest income (expense) Foreign Currency Transaction Gain (Loss), before Tax Foreign currency gain Foreign currency gain Other Nonoperating Income (Expense) Total other income (expense) Net Income (Loss) Attributable to Parent Net loss Net loss Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Other comprehensive income (loss): Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Foreign currency translation Comprehensive Income (Loss), Net of Tax, Attributable to Parent Total comprehensive loss Earnings Per Share, Basic and Diluted Net loss per share attributable to common stockholders, basic and diluted (in dollars per share) Weighted Average Number of Shares Outstanding, Basic and Diluted Weighted-average common shares outstanding, basic and diluted (in shares) Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders’ Deficit Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock [Member] Common Stock Additional Paid In Capital [Member] Additional Paid-in capital Accumulated Other Comprehensive Income [Member] Accumulated other comprehensive loss Retained Earnings [Member] Accumulated deficit Class of Stock [Axis] Class Of Stock [Domain] Class of Stock [Domain] Preferred stock that may be exchanged into common shares or other types of securities at the owner's option. Each share of Series A Preferred may be converted at any time, at the option of the holder, into a share of common stock. Convertible Preferred Stock Seriesa [Member] Series A convertible preferred stock Preferred stock that may be exchanged into common shares or other types of securities at the owner's option. Each share of Series B Preferred may be converted at any time, at the option of the holder, into a share of common stock. Convertible Preferred Stock Series B [Member] Series B convertible preferred stock Preferred stock that may be exchanged into common shares or other types of securities at the owner's option. Each share of Series C Preferred may be converted at any time, at the option of the holder, into a share of common stock. Convertible Preferred Stock Series C [Member] Series C convertible preferred stock Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity Temporary Equity, Carrying Amount, Including Portion Attributable to Noncontrolling Interests Temporary equity, ending balance Temporary equity, beginning balance Temporary Equity, Shares Outstanding Temporary Equity, Ending Balance (in shares) Temporary Equity, Beginning Balance (in shares) Temporary Equity, Stock Issued During Period, Value, New Issues Issuance of convertible preferred stock, net of issuance costs The number of new shares classified as temporary equity issued during the period. Temporary Equity, Stock Issued During Period, Shares, New Issues Issuance of convertible preferred stock (in shares) Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity Shares, Outstanding Equity, Ending Balance (in shares) Equity, Beginning Balance (in shares) Stock Issued During Period, Value, Stock Options Exercised Exercise of stock options Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Exercise of stock options (in shares) APIC, Share-based Payment Arrangement, Increase for Cost Recognition Stock based compensation Direct costs associated with issuing stock classified as temporary equity. Temporary Equity, Stock Issued, Issuance Costs Convertible preferred stock issuance costs Condensed Consolidated Statements of Cash Flows Net Cash Provided by (Used in) Operating Activities [Abstract] Cash flows from operating activities: Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net loss Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net loss to net cash used in operating activities: Depreciation, Depletion and Amortization Depreciation and amortization Employee Benefit and Share-based Payment Arrangement, Noncash Stock‑based compensation Amount of amortization expense attributable to right-of-use asset from operating leases. Operating Lease, Right-of-Use Asset, Amortization Amortization of right of use lease asset Increase (Decrease) in Operating Capital [Abstract] Changes in operating assets and liabilities: Increase (Decrease) in Prepaid Expense and Other Assets Prepaid expenses and other current assets Increase (Decrease) in Accounts Payable Accounts payable Increase (Decrease) in Accrued Liabilities Accrued expenses and other liabilities The increase (decrease) during the reporting period in operating lease liabilities. Increase (Decrease) In Operating Lease Liabilities Lease liability Net Cash Provided by (Used in) Operating Activities Net cash used in operating activities Net Cash Provided by (Used in) Investing Activities [Abstract] Cash flows from investing activities: Payments to Acquire Property, Plant, and Equipment Purchases of property and equipment Net Cash Provided by (Used in) Investing Activities Net cash used in investing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Cash flows from financing activities: Proceeds from Stock Options Exercised Proceeds from the exercise of stock options Payments of Debt Issuance Costs Deferred offering costs Proceeds from Issuance of Convertible Preferred Stock Proceeds from issuance of Series C convertible preferred, net Net Cash Provided by (Used in) Financing Activities Net cash provided by financing activities Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Increase (decrease) in cash and cash equivalents Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Effect of exchange rate changes on cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash and cash equivalents—end of year Cash and cash equivalents—beginning of year Noncash Investing and Financing Items [Abstract] Non‑cash activities: The noncash cumulative effect of changes in accounting principle during the reporting period. Noncash Cumulative Effect Of Change In Accounting Principle Cumulative effect of change in accounting principle—ASC 842 adoption Nature of business and basis of presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Nature of business and basis of presentation Summary of significant accounting policies Significant Accounting Policies [Text Block] Summary of significant accounting policies Leases The entire disclosure of information about leases. Lease Disclosure [Text Block] Leases Accrued expenses Accounts Payable and Accrued Liabilities Disclosure [Text Block] Accrued expenses Stockholders’ equity (deficit) Stockholders' Equity Note Disclosure [Text Block] Stockholders’ equity (deficit) Income Taxes Income Tax Disclosure [Text Block] Income Taxes Commitments and contingencies Commitments and Contingencies Disclosure [Text Block] Commitments and contingencies Consolidation, Policy [Policy Text Block] Principles of consolidation Disclosure of accounting policy for unaudited interim condensed consolidated financial statements. Unaudited Interim Condensed Consolidated Financial Statements [Policy Text Block] Unaudited interim condensed consolidated financial statements Use of Estimates, Policy [Policy Text Block] Use of estimates Foreign Currency Transactions and Translations Policy [Policy Text Block] Foreign currency and currency translation Cash and Cash Equivalents, Policy [Policy Text Block] Cash and cash equivalents Fair Value of Financial Instruments, Policy [Policy Text Block] Fair value of financial instruments Lessee, Leases [Policy Text Block] Accounting for leases Share-based Payment Arrangement [Policy Text Block] Stock based compensation Earnings Per Share, Policy [Policy Text Block] Net loss per share New Accounting Pronouncements, Policy [Policy Text Block] Recently issued accounting pronouncements Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Schedule of potentially dilutive securities excluded from calculation of diluted net loss per share Lease, Cost [Table Text Block] Summary of quantitative information regarding the Company’s leases Lessee, Operating Lease, Liability, Maturity [Table Text Block] Schedule of future lease payments under noncancellable leases Schedule of Accrued Liabilities [Table Text Block] Schedule of accrued expenses Convertible Preferred Stock [Member] Convertible preferred stock Cash Cash balance Ratio applied to the conversion of common shares of subsidiary for shares of parent entity. Conversion Of Common Stock, Conversion Ratio Conversion ratio Property, Plant and Equipment [Table] Property, Plant and Equipment [Line Items] Property and equipment Lease, Practical Expedients, Package [true false] Leases, election of practical expedients package Lessee, Operating Lease, Discount Rate Weighted average discount rate Incremental borrowing rate The table contains disclosure pertaining to an entity's basic and diluted earnings per share. Schedule of Earnings Per Share, Basic and Diluted [Table] Antidilutive Securities [Axis] Antidilutive Securities Name [Domain] Antidilutive Securities, Name [Domain] Employee Stock Option [Member] Options to purchase common stock Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Schedule of Earnings Per Share, Basic and Diluted [Line Items] Net loss per share n/a Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract] Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Total shares of common stock equivalents Lessee, Lease, Description [Table] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Operating Expense [Member] Operating Expenses Lessee, Lease, Description [Line Items] Leases The number of operating leases held by the Company. Lessee, Operating Lease, Number of Operating Leases Number of operating leases Operating Lease, Expense Rent expense - ASC 842 Lessee Disclosure [Abstract] Quantitative information regarding the Company’s leases Operating Lease, Cost Operating lease cost n/a Lessee, Other Lease Information [Abstract] Other Information Operating Lease, Payments Operating cash flows paid for amounts included in the measurement of lease liabilities Remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Operating Lease Remaining Lease Term Weighted average remaining lease term (years) Lessee, Operating Lease, Liability, Payment, Due [Abstract] Future Lease Payments Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year Remainder of 2020 Lessee, Operating Lease, Liability, Payments, Due Year Two 2021 Lessee, Operating Lease, Liability, Payments, Due Year Three 2022 Lessee, Operating Lease, Liability, Payments, Due Total Lease Payments Lessee, Operating Lease, Liability, Undiscounted Excess Amount Less: Imputed Interest Operating Lease, Liability Total Lease Liabilities Accrued Professional Fees, Current Professional fees Employee-related Liabilities, Current Compensation and benefits Total of the carrying values as of the balance sheet date of obligations incurred through that date and payable for obligations related to research and development costs. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). Research and Development Related Liabilities, Current Research and development Other Accrued Liabilities, Current Other Schedule of Stock by Class [Table] Class of Stock [Line Items] Stockholders’ equity (deficit) The combined maximum number of shares for all classes of equity permitted to be issued by an entity's charter and bylaws. Common Stock Equity And Temporary Equity, Authorized Shares Aggregate number of authorized shares for all classes of equity Preferred Stock, Shares Authorized Preferred stock, authorized Preferred Stock, Par or Stated Value Per Share Preferred stock, par value Number of voting rights per share of common stock owned. Common Stock, Number of Voting Rights Per Share Number of voting rights per common share Share-based Payment Arrangement, Expense Stock based compensation expense Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Line Items] Income taxes Income Tax Expense (Benefit) Income tax expense Unrecognized Tax Benefits Unrecognized tax benefits Loss Contingency Accrual, Provision Provision for contingent losses Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Subsidiaries [Member] Aprea Therapeutics AB Statistical Measurement [Axis] Range [Member] Statistical Measurement [Domain] Minimum [Member] Minimum Maximum [Member] Maximum