Organization and Description of Business	6 Months Ended
Organization and Description of Business	Jun. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of Business	Organization and Description of Business Description of Business PhaseBio Pharmaceuticals, Inc. (the “Company”) was incorporated as a Delaware corporation on January 10, 2002. The Company is a clinical-stage biopharmaceutical company focused on the development and commercialization of novel therapies for cardiopulmonary diseases. The Company’s lead product candidate, bentracimab (also known as PB2452), is a novel reversal agent for the antiplatelet drug ticagrelor, which the Company is developing for the reversal of the antiplatelet effects of ticagrelor in patients with uncontrolled major or life-threatening bleeding or requiring urgent surgery or an invasive procedure. The Company’s second product candidate, pemziviptadil (also known as PB1046), is in Phase 2 development for the treatment of pulmonary arterial hypertension ("PAH"). Pemziviptadil utilizes the Company’s proprietary half-life extending elastin-like polypeptide technology, which also serves as an engine for the Company’s preclinical pipeline. The Company is also developing its preclinical product candidate, PB6440, for treatment-resistant hypertension. Liquidity The Company has experienced net losses and negative cash flows from operations and, as of June 30, 2021, had an accumulated deficit of $316.8 million. The Company expects to continue to incur net losses for at least the next several years. As of June 30, 2021, the Company had cash and cash equivalents of $64.5 million and working capital of $72.3 million. In January 2020, the Company entered into a co-development agreement ("SFJ Agreement") with SFJ Pharmaceuticals X, Ltd., an SFJ Pharmaceuticals Group company ("SFJ"), pursuant to which SFJ provides funding and operational support for the clinical development of bentracimab. Management believes that its existing cash and cash equivalents as of June 30, 2021, in addition to the $12.5 million in clinical trial costs and other expenses that the Company expects SFJ will fund or reimburse pursuant to the SFJ Agreement and the $20.0 million received from Alfasigma S.p.A. ("Alfasigma") in July 2021 pursuant to the Alfasigma Sublicense (as described below), will be sufficient to fund operating expenses and capital requirements through the fourth quarter of 2022. In July 2021, the Company received a $20.0 million upfront payment upon entering into an exclusive sublicense agreement ("Alfasigma Sublicense") with Alfasigma for the commercialization of any product composed of or containing bentracimab (the "Licensed Products") in the European Union and European Economic Area, as well as the United Kingdom, Russia, Ukraine and certain other countries within the Commonwealth of Independent States, Europe and central Asia (the "Sublicense Territory"). The Company plans to address its future liquidity needs through the pursuit of additional funding through a combination of equity or debt financings, or other third-party financing, marketing and distribution arrangements and other collaborations, strategic alliances and licensing arrangements. However, there is no assurance that these funding efforts will be successful. The Company currently has an effective shelf registration statement on Form S-3 ("2019 Shelf Registration Statement") on file with the Securities and Exchange Commission ("SEC"), which expires in January 2023. The 2019 Shelf Registration Statement currently permits (i) the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $200.0 million of common stock, preferred stock, debt securities and warrants in one or more offerings and in any combination and (ii) the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $60.0 million of common stock that may be issued and sold under the "at-the-market" sales agreement with Citigroup Global Markets Inc. and William Blair & Company, L.L.C. (the "ATM Program"). As of June 30, 2021, the Company has sold 561,848 shares of common stock pursuant to the ATM Program for net proceeds of $2.9 million. During the six months ended June 30, 2021, the Company sold 18,400,000 shares of common stock in an underwritten offering pursuant to the 2019 Shelf Registration Statement for net proceeds of $60.2 million, after deducting underwriting discounts and commissions and other offering costs. As of June 30, 2021, the Company had $132.6 million of common stock remaining that can be sold under the 2019 Shelf Registration Statement, of which $57.0 million may be sold under the ATM Program. The Company is continuing to assess the effect that the COVID-19 pandemic may have on its business and operations. The extent to which COVID-19 may impact the Company's business and operations will depend on future developments that are highly uncertain and cannot be predicted with confidence, such as the geographic distribution of the disease and its variants over time, the efficacy, availability and pace of administration of vaccines and antiviral agents against the disease, the continued duration of the outbreak, the duration and effect of business disruptions and the short-term effects and ultimate effectiveness of the travel restrictions, quarantines, social distancing requirements and business closures in the United States and other countries to contain and treat the disease. While the potential economic impact brought by, and the continued duration of, the COVID-19 pandemic may be difficult to assess or predict, a continued and growing pandemic could result in significant disruption of global financial markets, reducing the Company's ability to access capital, which could in the future negatively affect its liquidity. In addition, a recession or market correction resulting from the spread of COVID-19 could materially affect the Company's business and the value of its common stock. Basis of Presentation The accompanying condensed financial statements have been prepared in accordance with United States generally accepted accounting principles (“GAAP”) for interim financial reporting and the rules and regulations of the SEC. Certain information and note disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to those rules and regulations. All adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of the accompanying condensed financial statements have been made. Although these interim condensed financial statements do not include all of the information and footnotes required for complete annual financial statements, management believes the disclosures are adequate to make the information presented not misleading. The unaudited interim results of operations and cash flows for the three and six months ended June 30, 2021 are not necessarily indicative of the results that may be expected for the full year. The unaudited interim condensed financial statements and footnotes should be read in conjunction with the audited financial statements and footnotes for the year ended December 31, 2020, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the SEC on March 15, 2021, wherein a more complete discussion of significant accounting policies and certain other information can be found. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) promulgated by the Financial Accounting Standards Board (“FASB”). The Company manages its operations as a single reportable segment for the purposes of assessing performance and making operating decisions.