Organization	3 Months Ended
Organization	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization Description of Business Prothena Corporation plc (“Prothena” or the “Company”) is a late-stage clinical company with a robust pipeline of novel investigational therapeutics built on protein dysregulation expertise with the potential to change the course of devastating rare peripheral amyloid and neurodegenerative diseases. Fueled by its deep scientific expertise built over decades of research, the Company is advancing a pipeline of therapeutic candidates for a number of indications and novel targets for which its ability to integrate scientific insights around neurological dysfunction and the biology of misfolded proteins can be leveraged. The Company’s wholly-owned programs include birtamimab for the potential treatment of AL amyloidosis, PRX004 for the potential treatment of ATTR amyloidosis, and a portfolio of programs for the potential treatment of Alzheimer’s disease including PRX012 that targets Aβ (Amyloid beta). The Company’s partnered programs include prasinezumab, in collaboration with Roche for the potential treatment of Parkinson’s disease and other related synucleinopathies, and programs that target tau (PRX005), TDP-43 and an undisclosed target in collaboration with Bristol Myers Squibb for the potential treatment of Alzheimer’s disease, amyotrophic lateral sclerosis (ALS). The Company was formed on September 26, 2012, under the laws of Ireland and re-registered as an Irish public limited company on October 25, 2012. The Company's ordinary shares began trading on The Nasdaq Global Market under the symbol “PRTA” on December 21, 2012, and currently trade on The Nasdaq Global Select Market. Liquidity and Business Risks As of March 31, 2021, the Company had an accumulated deficit of $819.8 million and cash and cash equivalents of $343.0 million. In March 2021, the Company sold an aggregate of 4,025,000 ordinary shares for net proceeds of approximately $78.1 million, after deducting the underwriting discount and estimated offering expenses, in an underwritten public offering. Based on the Company's business plans, management believes that the Company’s cash and cash equivalents at March 31, 2021, are sufficient to meet its obligations for at least the next twelve months. To operate beyond such period, or if the Company elects to increase its spending on research and development programs significantly above current long-term plans or enters into potential licenses and or other acquisitions of complementary technologies, products or companies, the Company may need additional capital. The Company expects to continue to finance future cash needs that exceed its cash from operating activities primarily through its current cash and cash equivalents, its collaborations with Roche and Bristol Myers Squibb, and, to the extent necessary, through proceeds from public or private equity or debt financings, loans and other collaborative agreements with corporate partners or other arrangements. The Company is subject to a number of risks, including but not limited to: the uncertainty of the Company’s research and development (“R&D”) efforts resulting in future successful commercial products; obtaining regulatory approval for its product candidates; its ability to successfully commercialize its product candidates, if approved; significant competition from larger organizations; reliance on the proprietary technology of others; dependence on key personnel; uncertain patent protection; dependence on corporate partners and collaborators; the outbreak of the novel strain of coronavirus SARS-CoV-2; and possible restrictions on reimbursement from governmental agencies and healthcare organizations, as well as other changes in the healthcare industry.