Organization and Description of Business	12 Months Ended
Organization and Description of Business	Dec. 31, 2020
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. Going Concern The Company has experienced net losses and negative cash flows from operations and, as of December 31, 2020, had an accumulated deficit of $260.7 million. The Company expects to continue to incur net losses for at least the next several years. As of December 31, 2020, the Company had cash and cash equivalents of $28.1 million and working capital of $25.2 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 December 31, 2020, in addition to the $42.9 million in clinical trial costs and other expenses that the Company expects SFJ will fund or reimburse pursuant to the SFJ Agreement, will not be sufficient to fund operating expenses and capital requirements for 12 months from the date of the issuance of these financial statements. These factors raise substantial doubt about the Company's ability to continue as a going concern. The financial statements do not include any adjustments that might result from the outcome of this uncertainty. Under the SFJ Agreement, if the Company fails to remedy such going concern condition during the periods specified in the agreement, SFJ may elect to have the Company's business related to bentracimab transferred to SFJ. The Company plans to address its 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. In this regard, 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 the offering, issuance and sale by the Company of up to an 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, of which $60.0 million may be offered, issued and sold in "at-the-market" sales pursuant to an equity distribution agreement with Citigroup Global Markets Inc. and William Blair & Company, L.L.C (the "ATM Program"). During the year ended December 31, 2020, the Company sold 561,848 shares of common stock pursuant to the ATM Program for net proceeds of $2.9 million. As of December 31, 2020, the Company had $197.1 million of common stock remaining that can be sold under the 2019 Shelf Registration Statement, of which $57.1 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 over time, the efficacy and availability 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, COVID-19 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 financial statements have been prepared in accordance with United States generally accepted accounting principles (“GAAP”) and the rules and regulations of the SEC. 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.