Nature of the Business and Basis of Presentation	12 Months Ended
Dec. 31, 2017
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Spero Therapeutics, Inc., together with its consolidated subsidiaries (the “Company”), is a multi-asset, clinical-stage biopharmaceutical company focused on identifying, developing and commercializing novel treatments for multi-drug resistant (“MDR”) bacterial infections. The Company’s most advanced product candidate, SPR994, is designed to be the first broad-spectrum oral carbapenem-class antibiotic for use in adults to treat MDR Gram-negative infections. Treatment with effective orally administrable antibiotics may prevent hospitalizations for serious infections and enable earlier, more convenient and cost-effective treatment of patients after hospitalization. The Company also has a platform technology known as its Potentiator Platform that it believes will enable it to develop drugs that will expand the spectrum and potency of existing antibiotics, including formerly inactive antibiotics, against Gram-negative bacteria. The Company’s lead product candidates generated from its Potentiator Platform are two intravenous, or IV,-administered agents, SPR741 and SPR206, designed to treat MDR Gram-negative infections in the hospital setting. In addition, the Company is developing SPR720, an oral antibiotic designed for the treatment of pulmonary non-tuberculous mycobacterial infections. The Company believes that its novel product candidates, if successfully developed and approved, would have a meaningful patient impact and significant commercial applications for the treatment of MDR infections in both the community and hospital settings. The Company was formed as Spero Therapeutics, LLC in December 2013 under the laws of the State of Delaware. On June 30, 2017, through a series of transactions, Spero Therapeutics, LLC merged with and into Spero Therapeutics, Inc. (formerly known as Spero OpCo, Inc.), a Delaware corporation. As part of the transactions, holders of preferred units and common units of Spero Therapeutics, LLC exchanged their units for shares of Spero Therapeutics, Inc. on a one-for-one basis. These transactions are collectively referred to as the Reorganization. Upon completion of the Reorganization, the historical consolidated financial statements of Spero Therapeutics, LLC became the historical consolidated financial statements of Spero Therapeutics, Inc. because the Reorganization was accounted for as a reorganization of entities under common control. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On October 20, 2017, the Company effected a one-for-6.0774 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Preferred Stock (see Note 5). Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. In addition, all common units and incentive units as well as the conversion ratios of preferred units of Spero Therapeutics, LLC have been presented as if the reverse stock split of the common stock of Spero Therapeutics, Inc. had been applied to such units and ratios of Spero Therapeutics, LLC. On November 6, 2017, Spero Therapeutics, Inc. completed an initial public offering (“IPO”) of its common stock, and issued and sold 5,500,000 shares of common stock at a public offering price of $14.00 per share, resulting in net proceeds of $71.6 million after deducting underwriting discounts and commissions but before deducting offering costs. On November 14, 2017, Spero Therapeutics, Inc., issued and sold an additional 471,498 shares of its common stock at the IPO price of $14.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $6.1 million after deducting underwriting discounts and commissions. Upon the closing of the IPO in November 2017, the Company’s outstanding convertible preferred shares automatically converted into shares of common stock (see Note 6). The accompanying consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its consolidated subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. In accordance with Accounting Standards Update (“ASU”) 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. Since inception, the Company has funded its operations with proceeds from sales of preferred units (including bridge units, which converted into preferred units), payments received in connection with a concluded collaboration agreement and funding from government contracts, and most recently, with proceeds from the IPO completed in November 2017. The Company has incurred recurring losses since inception, including net losses attributable to Spero Therapeutics, Inc. of $38.7 million, $25.5 million and $10.2 million for the years ended December 31, 2017, 2016 and 2015, respectively. In addition, as of December 31, 2017, the Company had an accumulated deficit of $96.8 million. The Company expects to continue to generate operating losses for the foreseeable future. As of the issuance date of the annual consolidated financial statements, the Company expects that its cash and cash equivalents, would be sufficient to fund its operating expenses, capital expenditure requirements through at least 12 months from the issuance date of these annual consolidated financial statements. However, the future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its future operations. The Company will seek additional funding through public or private financings, debt financing, collaboration agreements or government grants. The inability to obtain funding, as and when needed, would have a negative impact on the Company’s financial condition and ability to pursue its business strategies. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. Although management intends to pursue plans to obtain additional funding to finance its operations, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. The accompanying consolidated financial statements do not include any adjustments that might result from the outcome of this uncertainty. Accordingly, the consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business.

1. Nature of the Business and Basis of Presentation

Spero Therapeutics, Inc., together with its consolidated subsidiaries (the “Company”), is a multi-asset, clinical-stage biopharmaceutical company focused on identifying, developing and commercializing novel treatments for multi-drug resistant (“MDR”) bacterial infections. The Company’s most advanced product candidate, SPR994, is designed to be the first broad-spectrum oral carbapenem-class antibiotic for use in adults to treat MDR Gram-negative infections. Treatment with effective orally administrable antibiotics may prevent hospitalizations for serious infections and enable earlier, more convenient and cost-effective treatment of patients after hospitalization. The Company also has a platform technology known as its Potentiator Platform that it believes will enable it to develop drugs that will expand the spectrum and potency of existing antibiotics, including formerly inactive antibiotics, against Gram-negative bacteria. The Company’s lead product candidates generated from its Potentiator Platform are two intravenous, or IV,-administered agents, SPR741 and SPR206, designed to treat MDR Gram-negative infections in the hospital setting. In addition, the Company is developing SPR720, an oral antibiotic designed for the treatment of pulmonary non-tuberculous mycobacterial infections. The Company believes that its novel product candidates, if successfully developed and approved, would have a meaningful patient impact and significant commercial applications for the treatment of MDR infections in both the community and hospital settings.

The Company was formed as Spero Therapeutics, LLC in December 2013 under the laws of the State of Delaware. On June 30, 2017, through a series of transactions, Spero Therapeutics, LLC merged with and into Spero Therapeutics, Inc. (formerly known as Spero OpCo, Inc.), a Delaware corporation. As part of the transactions, holders of preferred units and common units of Spero Therapeutics, LLC exchanged their units for shares of Spero Therapeutics, Inc. on a one-for-one basis. These transactions are collectively referred to as the Reorganization. Upon completion of the Reorganization, the historical consolidated financial statements of Spero Therapeutics, LLC became the historical consolidated financial statements of Spero Therapeutics, Inc. because the Reorganization was accounted for as a reorganization of entities under common control.

The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

On October 20, 2017, the Company effected a one-for-6.0774 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Preferred Stock (see Note 5). Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. In addition, all common units and incentive units as well as the conversion ratios of preferred units of Spero Therapeutics, LLC have been presented as if the reverse stock split of the common stock of Spero Therapeutics, Inc. had been applied to such units and ratios of Spero Therapeutics, LLC.

On November 6, 2017, Spero Therapeutics, Inc. completed an initial public offering (“IPO”) of its common stock, and issued and sold 5,500,000 shares of common stock at a public offering price of $14.00 per share, resulting in net proceeds of $71.6 million after deducting underwriting discounts and commissions but before deducting offering costs. On November 14, 2017, Spero Therapeutics, Inc., issued and sold an additional 471,498 shares of its common stock at the IPO price of $14.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $6.1 million after deducting underwriting discounts and commissions. Upon the closing of the IPO in November 2017, the Company’s outstanding convertible preferred shares automatically converted into shares of common stock (see Note 6).

The accompanying consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its consolidated subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.

In accordance with Accounting Standards Update (“ASU”) 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. Since inception, the Company has funded its operations with proceeds from sales of preferred units (including bridge units, which converted into preferred units), payments received in connection with a concluded collaboration agreement and funding from government contracts, and most recently, with proceeds from the IPO completed in November 2017. The Company has incurred recurring losses since inception, including net losses attributable to Spero Therapeutics, Inc. of $38.7 million, $25.5 million and $10.2 million for the years ended December 31, 2017, 2016 and 2015, respectively. In addition, as of December 31, 2017, the Company had an accumulated deficit of $96.8 million. The Company expects to continue to generate operating losses for the foreseeable future. As of the issuance date of the annual consolidated financial statements, the Company expects that its cash and cash equivalents, would be sufficient to fund its operating expenses, capital expenditure requirements through at least 12 months from the issuance date of these annual consolidated financial statements. However, the future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its future operations. The Company will seek additional funding through public or private financings, debt financing, collaboration agreements or government grants. The inability to obtain funding, as and when needed, would have a negative impact on the Company’s financial condition and ability to pursue its business strategies. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. Although management intends to pursue plans to obtain additional funding to finance its operations, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

The accompanying consolidated financial statements do not include any adjustments that might result from the outcome of this uncertainty. Accordingly, the consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business.