| Summary of Significant Accounting Policies | Summary of Significant Accounting Policies Basis of Preparation and Presentation of Financial Information These Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the U.S. (“GAAP”) and with the instructions for Form 10-K and Regulations S-X statements. The Consolidated Financial Statements of Prothena Corporation plc are presented in U.S. dollars, which is the functional currency of the Company. These Consolidated Financial Statements include the accounts of the Company and its consolidated subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Certain amounts in the Consolidated Financial Statements have been reclassified to conform to the current year presentation. Use of Estimates The preparation of the Consolidated Financial Statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosures. On an ongoing basis, management evaluates its estimates, including critical accounting policies or estimates related to revenue recognition, share-based compensation, research and development expenses and leases. The Company bases its estimates on historical experience and on various other market specific and other relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Because of the uncertainties inherent in such estimates, actual results may differ materially from these estimates. Significant Accounting Policies Cash and Cash Equivalents The Company considers all highly liquid investments held at financial institutions, such as commercial paper, money market funds, and other money market securities with original maturities of three months or less at date of purchase to be cash equivalents. Restricted Cash Cash accounts that are restricted to withdrawal or usage are presented as restricted cash. As of December 31, 2019 , the Company had $2.7 million of restricted cash held by a bank in a certificate of deposit as collateral to a standby letter of credit under an operating lease. This amount is classified as a non-current asset in the Company's Consolidated Balance Sheet. See Note 6, "Commitments and Contingencies" for additional information regarding our operating lease. Property and Equipment, net Property and equipment, net are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the related assets. Maintenance and repairs are charged to expense as incurred, and improvements and betterments are capitalized. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized. Depreciation and amortization periods for the Company’s property, plant and equipment are as follows: Useful Life Machinery and equipment 4-7 years Leasehold improvements Shorter of expected useful life or lease term Purchased computer software 4 years Impairment of Long-lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable or the estimated useful life is no longer appropriate. If circumstances require that a long-lived asset be tested for possible impairment, the Company compares the undiscounted cash flows expected to be generated by the asset to the carrying amount of the asset. If the carrying amount of the long-lived asset is not recoverable on an undiscounted cash flow basis, an impairment is recognized to the extent that the carrying amount exceeds its fair value. The Company determines fair value using the income approach based on the present value of expected future cash flows. The Company’s cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors. On October 30, 2018, the Company entered into a surrender agreement for its office space in Dún Laoghaire, Ireland. The Company paid €270,000 , or $309,000 as converted using an exchange rate as of November 28, 2018, as full and final settlement of outstanding contractual obligations of $1.6 million in exchange for surrender and assignment to the landlord including surrender of approximately $0.5 million of long-lived assets, which was recorded as an asset impairment in the year ended December 31, 2018. There were no impairment charges recorded during the years ended December 31, 2019 , and 2017 . Leases At the inception, the Company determines if an arrangement is a lease. If so, the Company evaluates the lease agreement to determine whether the lease is an operating or capital using the criteria in ASC 842. The Company does not recognize right-of-use assets and lease liabilities that arise from short-term leases for any class of underlying assets. When lease agreements also require the Company to make additional payments for taxes, insurance and other operating expenses incurred during the lease period, such payments are expensed as incurred. See Note 6, “Commitments and Contingencies,” which provides additional details on the Company's current lease arrangements. Operating Leases Operating leases are included in the operating lease right-of-use assets, lease liability, current and lease liability, non-current in the Company's Consolidated Balance Sheets. Operating lease right-of-use assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of minimum lease payments over the lease term. In determining the present value of lease payments, the Company uses its incremental borrowing rate based on information available at the lease commencement date. The operating lease right-of-use assets also include any lease prepayments made and exclude lease incentives including rent abatements and/or concessions and rent holidays. Tenant improvements made by the Company as a lessee in which they are deemed to be owned by the lessor is viewed as lease prepayments by the Company and included in the operating lease right-of-use assets. Lease expense is recognized on a straight-line basis over the expected lease term. For lease agreements entered after the adoption of ASC 842 that include lease and non-lease components, such components are generally accounted separately. Revenue Recognition Revenue is recognized only when the Company satisfies an identified performance obligation by transferring a promised good or service to a customer. Contracts with Multiple Performance Obligations The Company’s License Agreement with Roche contains multiple performance obligations. The Company accounts for the individual performance obligations separately if they are distinct. Factors considered in the determination of whether the license performance obligations are distinct included, among other things, the research and development capabilities of Roche and Roche’s sublicense rights, and for the remaining performance obligations the fact that they are not proprietary and can be and have been provided by other vendors. The transaction price is allocated to the separate performance obligation on a relative standalone selling price basis. The Company does not disclose the value of unsatisfied performance obligations for (i) contracts with an original expected length of one year or less and (ii) contracts for which the Company recognize revenue at the amount to which the Company has the right to invoice for services performed. Collaboration Revenue Upon adoption of ASC 606, the Company recognizes research and development reimbursements as collaboration revenue earned over time as services are performed. Prior to adoption of ASC 606, the Company recorded research reimbursement as collaboration revenue and development reimbursement as an offset to R&D expense once the license revenue cap was met. Milestone Revenue The Company generally classifies each of its milestones into one of three categories: (i) clinical milestones; (ii) regulatory and development milestones; and (iii) commercial milestones. Clinical milestones are typically achieved when a product candidate advances into or completes a defined phase of clinical research. For example, a milestone payment may be due to the Company upon the initiation of a clinical trial for a new indication. Regulatory and development milestones are typically achieved upon acceptance of the submission for marketing approval of a product candidate or upon approval to market the product candidate by the FDA or other regulatory authorities. For example, a milestone payment may be due to the Company upon submission for marketing approval of a product candidate by the FDA. Commercial milestones are typically achieved when an approved product reaches certain defined levels of net royalty sales by the licensee of a specified amount within a specified period. In general, the Company considers such milestone payments as variable consideration with constraint and therefore recognizes the revenue from such milestone payments as collaboration revenue at point in time when the Company can conclude it is probable that a significant revenue reversal will not occur in future periods. Profit Share Revenue For agreements, with profit sharing arrangements, the Company will record its share of the pre-tax commercial profit as collaboration revenue when the profit sharing can be reasonably estimated and that a significant revenue reversal will not occur in future periods. Royalty Revenue The Company will recognize revenue from royalties based on licensees' sales of the Company's products or products using the Company's technologies. Royalties are recognized as earned in accordance with the contract terms when royalties from licensees can be reasonably estimated and that a significant revenue reversal will not occur in future periods. There were no royalties earned during the years ended December 31, 2019, 2018 and 2017 . Taxes, Shipping and Handling The Company excludes from the measurement of the transaction price all taxes assessed by a governmental authority that are both imposed on and concurrent with a specific revenue-producing transaction and collected by the Company from a customer (e.g., sales, use, value added, some excise taxes). In addition, the Company accounts for shipping and handling as activities that are performed after its customers obtain control of the goods as activities to fulfill our performance obligation to transfer the goods. Incremental Costs to Obtain or Fulfill a Contract For costs to obtain a contract, the Company will capitalize such amounts if they are incremental and expected to be recovered. Sales commissions directly related to obtaining new contracts will be capitalized unless the amortization period is one year or less, at which these costs will be recorded within selling and general administrative expenses. As of December 31, 2019 , the Company does not have such costs capitalized in its Consolidated Balance Sheet. Research and Development Research and development costs are expensed as incurred and include, but are not limited to, salary and benefits, share-based compensation, clinical trial activities, drug development and manufacturing prior to FDA and other regulatory approval and third-party service fees, including clinical research organizations and investigative sites. Costs for certain development activities, such as clinical trials, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by its vendors on their actual costs incurred. The objective of the Company’s accrual policy is to match the recording of the expenses in its Consolidated Financial Statements to the actual services received and efforts expended. As such, expense accruals related to clinical trials are recognized based on its estimate of the degree of completion of the events specified in the specific clinical study or trial contract. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the Consolidated Financial Statements as prepaid or accrued research and development. Amounts due may be fixed fee, fee for service, and may include upfront payments, monthly payments, and payments upon the completion of milestones or receipt of deliverables. Acquired In-Process Research and Development Expense The Company has acquired and may continue to acquire the rights to develop and commercialize new drug candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately expensed as research and development provided that the drug has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. Restructuring Charges The Company recognizes restructuring charges related to its reorganization plan. In connection with these activities, the Company records restructuring charges for contractual employee termination benefits, one-time employee termination benefits and contract termination costs. The Company accounts for its restructuring charges as a liability when the obligations are incurred and records such charges at fair value. The recognition of restructuring charges requires the Company to make certain judgments and estimates regarding the nature, timing and amount of costs associated with the planned reorganization plan. To the extent the Company’s actual results differ from its estimates and assumptions, the Company may be required to revise the estimates of future liabilities, requiring the recognition of additional restructuring charges or the reduction of liabilities already recognized. Such changes to previously estimated amounts may be material to the Consolidated Financial Statements. Changes in the estimates of the restructuring charges are recorded in the period the change is determined. At the end of each reporting period, the Company evaluates the remai ning accrued balances to ensure that no excess accruals are retained and the utilization of the provisions are for their intended purpose in accordance with developed restructuring plans. See Note 11, “Restructuring” for additional information regarding restructuring charges. Loss Contingencies Loss contingencies, including claims and legal actions arising in the ordinary course of business, are recorded as liabilities when the likelihood of loss is probable and an amount or range of loss can be reasonably estimated. The Company's accruals for losses are based on management's judgment of all possible outcomes and their financial effect, the probability of losses, and where applicable, the consideration of opinions of the Company's legal counsel. The Company’s accounting policy for legal costs related to loss contingencies is to accrue for the probable fees that can be reasonably estimated and expensed as incurred. Additionally, the Company records insurance recovery receivable from third party insurers when recovery has been determined to be probable. Share-based Compensation To determine the fair value of share-based payment awards, the Company uses the Black-Scholes option-pricing model. The determination of fair value using the Black-Scholes option-pricing model is affected by the Company’s share price as well as assumptions regarding a number of complex and subjective variables. Judgment is required in determining the proper assumptions used in these models. The assumptions used include the risk-free interest rate, expected term, expected volatility and expected dividend yield. Share-based awards, including stock options, are measured at fair value as of the grant date and share-based compensation expense is recognized on a straight-line basis over the requisite service period for each award. Further, share-based compensation expense recognized in the Consolidated Statements of Operations is based on awards expected to vest and therefore the amount of expense has been reduced for estimated forfeitures. Forfeitures are estimated based on historical experience. If actual forfeitures differ from estimates at the time of grant they will be revised in subsequent periods. The Company uses its historical volatility for the Company's stock to estimate expected volatility. Through December 31, 2017, the expected volatility was based on a combination of historical volatility for the Company's stock and the historical volatilities of several of the Company's publicly traded comparable companies. If factors change and different assumptions are employed in determining the fair value of share-based awards, the share-based compensation expense recorded in future periods may differ significantly from what was recorded in the current period (see Note 9, "Share-Based Compensation" for further information). The Company records any excess tax benefits or tax shortfalls from its equity awards in its Consolidated Statements of Operations in the reporting periods in which stock options are exercised. Income Taxes The Company files its own U.S. and foreign income tax returns and income taxes are presented in the Consolidated Financial Statements using the asset and liability method prescribed by the accounting guidance for income taxes. Deferred tax assets (“DTAs”) and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using the enacted tax rates projected to be in effect for the year in which the differences are expected to reverse. Net deferred tax assets are recorded to the extent the Company believes that these assets will more likely than not be realized. In making such determination, all available positive and negative evidence is considered, including scheduled reversals of deferred tax liabilities, recent cumulative earnings/losses by taxing jurisdiction, projected future taxable income, tax planning strategies and recent financial operations. Actual operating results in future years could differ from our current assumptions, judgments and estimates. Our significant tax jurisdictions are Ireland and the United States. Estimates are required in determining the Company’s provision for income taxes. Some of these estimates are based on management’s interpretations of jurisdiction-specific tax laws or regulations. Various internal and external factors may have favorable or unfavorable effects on the future effective income tax rate of the business. These factors include, but are not limited to, changes in tax laws, regulations and/or rates, changing interpretations of existing tax laws or regulations, changes in estimates of prior years’ items, past and future levels of R&D spending, the impact of accounting for share-based compensation, and changes in overall levels of income before taxes. The Company did not recognize certain tax benefits from uncertain tax positions within the provision for income taxes. The tax benefit from an uncertain tax position is recognized only if it is more likely than not the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized upon settlement. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Interest and penalties related to unrecognized tax benefits are accounted for in income tax expense. Net Income (Loss) per Ordinary Share Basic net income (loss) per ordinary share is computed by dividing net income (loss) attributable to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by giving effect to all dilutive potential ordinary shares including options. However, potentially issuable ordinary shares are not used in computing diluted net loss per ordinary share as their effect would be anti-dilutive due to the loss recorded. In this case, diluted net loss per share is equal to basic net loss per share. Comprehensive Loss Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income (loss). The Company has no components of other comprehensive income (loss). Therefore net income (loss) equals comprehensive income (loss) for all periods presented and, accordingly, the Consolidated Statements of Comprehensive Income (Loss) is not presented in a separate statement. Segment and Concentration of Risks The Company operates in one segment. The Company’s chief operating decision maker (the “CODM”), its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources. When evaluating the Company’s financial performance, the CODM reviews all financial information on a consolidated basis. Financial instruments that potentially subject the Company to concentration of credit risk consist of cash and cash equivalents and accounts receivable. The Company places its cash equivalents with high credit quality financial institutions and by policy, limits the amount of credit exposure with any one financial institution. Deposits held with banks may exceed the amount of insurance provided on such deposits. The Company has not experienced any losses on its deposits of cash and cash equivalents and its credit risk exposure is up to the extent recorded on the Company's Consolidated Balance Sheet. The receivable from Roche recorded in prepaid expenses and other current assets in the Consolidated Balance Sheet are amounts due from a Roche entity located in Switzerland under the License Agreement that became effective January 22, 2014. Revenue recorded in the Consolidated Statements of Operations consists of reimbursement from Roche for research and development services. The Company's credit risk exposure is up to the extent recorded on the Company's Consolidated Balance Sheet. As of December 31, 2019 , $3.9 million of the Company’s property and equipment, net were held in the U.S. and none were in Ireland. As of December 31, 2018 , $52.8 million of the Company's property and equipment, net were held in the U.S. and none were in Ireland. The December 31, 2018, balance included building recorded under build to suit accounting that was derecognized upon the adoption of ASC 842 in 2019. The Company does not own or operate facilities for the manufacture, packaging, labeling, storage, testing or distribution of nonclinical or clinical supplies of any of its drug candidates. The Company instead contracts with and relies on third-parties to manufacture, package, label, store, test and distribute all preclinical development and clinical supplies of our drug candidates, and it plans to continue to do so for the foreseeable future. The Company also relies on third-party consultants to assist in managing these third-parties and assist with its manufacturing strategy. Recently Adopted Accounting Pronouncement In August 2018, the SEC issued Final Rule 33-10532, which updates and simplifies certain disclosure requirements. The rule was effective for filings on or after November 5, 2018. However, the SEC released guidance advising it will not object to a registrant adopting the requirement to include changes in stockholders' equity in the Form 10-Q for the first quarter beginning after the effective date of the rule (e.g. for a calendar year-end company, the first quarter of fiscal year 2019). The following amendments from the Final Rule 33-10532 are applicable to the Company: (1) an analysis of changes in stockholders' equity will now be required for the current and comparative year-to-date interim periods; and (2) for market price information, a registrant will disclose the ticker symbol of its common equity instead of disclosure of the high and low trading prices of an entity's common stock for specified quarterly periods. The Company's disclosure reflects the applicable amendments. In February 2016, the FASB issued Accounting Standards Update 2016-02 Topic 842, Leases ("ASC 842"), which requires lessees to recognize assets and liabilities for leases with lease terms of more than 12 months and disclose key information about leasing arrangements. ASC 842 was subsequently amended by ASU 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU 2018-10, Codification Improvements to Topic 842, Leases; ASU 2018-11, Targeted Improvements; ASU 2018-20, Narrow-Scope Improvements for Lessors; and ASU 2019-01, Codification Improvements. Under the new standard, a lessee will recognize liabilities on the balance sheet, initially measured at the present value of the lease payments, and right-of-use (ROU) assets representing its right to use the underlying asset for the lease term. For leases with a term of 12 months or less at the commencement date, a lessee is permitted to make an accounting policy election not to recognize lease assets and lease liabilities. The new standard also eliminates the previous build-to-suit lease accounting guidance, which results in the derecognition of build-to-suit assets and liabilities that remained on the balance sheet after the end of the construction period. The recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee primarily will depend on its classification as a finance or operating lease. The new guidance requires both types of leases to be recognized on the balance sheet. The Company adopted the new standard on January 1, 2019, using the modified retrospective transition method wherein the effective date is its date of initial application. Consequently, prior period amounts are not adjusted and continue to be reported in accordance with the Company’s historical accounting under ASC 840. The new standard provides a number of optional practical expedients in transition. The Company elected the "package of practical expedients", which permitted the Company not to reassess under the new standard its prior conclusions about lease identification, lease classification and initial direct cost. The Company did not elect the use-of-hindsight or the practical expedient pertaining to land easements; the latter not being applicable to the Company. For the Company's build-to-suit lease, Prothena has historically excluded executory costs, when part of the fixed payments in a lease contract, as part of the minimum rental payment disclosed in its financial statements footnote for the Current SSF Facility lease under ASC 840. Executory cost of a lease includes costs of taxes, insurance and maintenance (including common area maintenance). With the selection of practical expedient, the Company believes it is appropriate to continue applying the same accounting policy with its transition to ASC 842 (i.e. exclude the executory cost in determining the minimum rental payment). As of January 1, 2019, the Company recorded $3.8 million change to the opening balance of the accumulated deficit for the cumulative effect of applying ASC 842, which included a reduction of $1.0 million in deferred tax assets. See Note 6, “Commitments and Contingencies,” which provides additional details on the Company's current lease arrangements. The impact of the adoption of ASC 842 on the accompanying Consolidated Balance Sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments due to the Adoption of Topic 842 January 1, 2019 Property and equipment, net $ 52,835 $ (47,859 ) $ 4,976 Operating lease right-of-use assets $ — $ 28,530 $ 28,530 Deferred tax assets $ 9,702 $ (994 ) $ 8,708 Lease liability, current $ — $ 4,717 $ 4,717 Other current liabilities (1) $ 5,926 $ (44 ) $ 5,882 Build-to-suit lease obligation, current $ 1,645 $ (1,645 ) $ — Lease liability, non-current $ — $ 22,939 $ 22,939 Build-to-suit lease obligation, non-current $ 49,901 $ (49,901 ) $ — Deferred rent, non-current $ 176 $ (176 ) $ — Accumulated deficit $ (597,995 ) $ 3,787 $ (594,208 ) __________________ (1) Amount as of December 31, 2018, included deferred rent, current. The adjustments due to the adoption of ASC 842 relate to (1) the change in classification of build-to-suit lease under ASC 840 for the Company's current facility in South San Francisco, California to an operating lease under ASC 842 and as a result the Company derecognized its build-to-suit asset of $47.9 million under property and equipment, net as of December 31, 2018 and related liability of $51.5 million , and (2) recognized an operating lease right-of-use asset of $28.5 million and operating lease liability of $27.7 million on the Consolidated Balance Sheet for the Company's operating lease. The right-of-use asset includes tenant improvements added by the Company wherein the lessor was deemed the accounting owner, net of tenant improvement allowance paid by the lessor. The Company has no debt and has not had an established incremental borrowing rate. For the purpose of estimating the incremental borrowing rate in the adoption of ASC 842, the Company inquired with banks that had business relationship with the Company to determine the Company's collateralized incremental borrowing rate. The discount rate used to determine the lease liability was 4.25% . There is no change in the accounting of the Sub-Sublease of the Current SSF Facility upon adoption of ASC 842. Further, the Company's operating lease at Dublin is not included in the lease liability and right-of-use asset recorded due to its nominal amount. For the purpose of the adoption of ASC 842, the Company also performed an evaluation of its other contracts with customers and suppliers in accordance with ASC 842 and determined that, except for the office leases described in Note 6, “Commitments and Contingencies” (a nominal operating lease for medical monitoring equipment and a nominal operating lease for office equipment), none of the Company’s contracts contain a lease. Recent Accounting Pronouncements In November 2018, the FASB issued Accounting Standards Update 2018-18 ("ASU 2018-18"), Collaborative Arrangements: Clarifying the Interaction between Topic 808 and Topic 606, which clarifies when transactions between collaborative arrangement participants are in the scope of ASC 606 and provides some guidance on presentation of transactions not in the scope of ASC 606. This ASU is effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2019. Early adoption is permitted as long as entities have already adopted the guidance in ASC 606. The Company does not expect the adoption of ASU 2018-18 to have an impact on its consolidated financial statements. The Company will continue to evaluate the impact of ASU 2018-18 on its consolidated financial statements in connection with Roche License Agreement and Celgene Collaboration Agreement. On December 18, 2019 the FASB issued Accounting Standards Update 2019-12 ("ASU 2019-12"), Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which simplifies the accounting for income taxes as part of the Board’s overall initiative to reduce complexity in accounting standards. Amendments include removal of certain exceptions to the general principles of ASC 740, Income Taxes , and simplification in several other areas such as accounting for a franchise tax (or similar tax) that is partially based on income. While not required to be adopted until 2021 for most calendar year public business entities (and 2022 for other entities), early adoption is permitted for any financial statements. The Company does not expect the adoption of ASU 2019-12 to have a significant impact on its consolidated financial statements. The Company will continue to evaluate the impact of ASU 2019-12 on its consolidated financial statements. |
