Convertible Preferred Shares	9 Months Ended
Sep. 30, 2018
Equity [Abstract]
Convertible Preferred Shares	5.Convertible Preferred Shares Series A Convertible Preferred Shares The Company’s amended and restated certificate of incorporation authorizes its Board of Directors to issue up to 10,000,000 shares of preferred stock, par value $0.001 per share. As part of the Company’s July 2018 underwritten public offering, 2,220 shares were designated as Series A Convertible Preferred Stock and issued at a price of $12,500 per share. Each share of Series A Convertible Preferred Stock is convertible into 1,000 shares of the Company’s common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series A Convertible Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99% of the total number of shares of the Company’s common stock then issued and outstanding. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series A Convertible Preferred Stock will receive a payment equal to $0.001 per share of Series A Convertible Preferred Stock, plus an additional amount equal to any dividends declared but unpaid on such shares, before any proceeds are distributed to the holders of common stock or any of our securities that by their terms are junior to the Series A Convertible Preferred Stock. The Series A Convertible Preferred Stock has no voting rights, except as required by law and except that the consent of the outstanding Series A Convertible Preferred Stock holders will be required to amend the terms of the Series A Convertible Preferred Stock. The Series A Convertible Preferred Stock does not have any mandatory redemption rights or other redemption rights that would be outside of the Company’s control. As such, the Company has classified the Series A Convertible Preferred Stock within permanent equity in its consolidated balance sheet. Redeemable Convertible Preferred Shares Prior to the Reorganization, the operating agreement of Spero Therapeutics, LLC, as amended and restated, provided for the issuance of Junior preferred units, Class A preferred units, Class B preferred units and bridge units, but did not specify an authorized number of each for issuance. Subsequent to the Company’s Reorganization on June 30, 2017 (see Note 1), the Company’s amended and restated certificate of incorporation authorized the issuance of 43,297,267 shares of preferred stock, par value $0.001 per share, and holders of outstanding preferred units of Spero Therapeutics, LLC exchanged their units for preferred stock of Spero Therapeutics, Inc. on a one-for-one basis. The rights and preferences of each class of stock were the same both before and after the Reorganization. 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 rations for each series or the Company’s Preferred Stock. Upon the closing of the Company’s IPO in November 2017, all of the then outstanding convertible preferred shares automatically converted into shares of common stock. In March 2017, the Company issued and sold 24,326,470 Class C preferred units at a price of $1.7749 per unit for proceeds of $43.0 million, net of issuance costs of $0.2 million. The sale of Class C preferred units met the definition of a qualified financing under the 2016 bridge unit agreements. The Company issued 5,321,112 Class C preferred units upon the conversion of the 2016 bridge units in the amount of $8.5 million, at a conversion price of $1.60 per unit, which represented a discount of 10% to the price per unit paid by other investors in the Class C preferred unit financing. The conversion was accounted for as an extinguishment for accounting purposes. Accordingly, the Company recorded the Class C preferred units issued upon conversion of the 2016 bridge units at their aggregate fair value of $9.4 million and recorded a corresponding adjustment to extinguish the then-current carrying value of the 2016 bridge units of $8.5 million and the then-current fair value of the derivative liability related to the contingent prepayment option associated with the 2016 bridge units of $0.9 million. There was no gain or loss recognized upon the extinguishment. In July 2017 the Company sold to its Chief Financial Officer 61,880 shares of the Company’s Series C preferred stock at a price of $1.7749 per share, for proceeds of $0.1 million.

5.Convertible Preferred Shares

Series A Convertible Preferred Shares

The Company’s amended and restated certificate of incorporation authorizes its Board of Directors to issue up to 10,000,000 shares of preferred stock, par value $0.001 per share. As part of the Company’s July 2018 underwritten public offering, 2,220 shares were designated as Series A Convertible Preferred Stock and issued at a price of $12,500 per share.

Each share of Series A Convertible Preferred Stock is convertible into 1,000 shares of the Company’s common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series A Convertible Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99% of the total number of shares of the Company’s common stock then issued and outstanding. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series A Convertible Preferred Stock will receive a payment equal to $0.001 per share of Series A Convertible Preferred Stock, plus an additional amount equal to any dividends declared but unpaid on such shares, before any proceeds are distributed to the holders of common stock or any of our securities that by their terms are junior to the Series A Convertible Preferred Stock. The Series A Convertible Preferred Stock has no voting rights, except as required by law and except that the consent of the outstanding Series A Convertible Preferred Stock holders will be required to amend the terms of the Series A Convertible Preferred Stock. The Series A Convertible Preferred Stock does not have any mandatory redemption rights or other redemption rights that would be outside of the Company’s control. As such, the Company has classified the Series A Convertible Preferred Stock within permanent equity in its consolidated balance sheet.

Redeemable Convertible Preferred Shares

Prior to the Reorganization, the operating agreement of Spero Therapeutics, LLC, as amended and restated, provided for the issuance of Junior preferred units, Class A preferred units, Class B preferred units and bridge units, but did not specify an authorized number of each for issuance. Subsequent to the Company’s Reorganization on June 30, 2017 (see Note 1), the Company’s amended and restated certificate of incorporation authorized the issuance of 43,297,267 shares of preferred stock, par value $0.001 per share, and holders of outstanding preferred units of Spero Therapeutics, LLC exchanged their units for preferred stock of Spero Therapeutics, Inc. on a one-for-one basis. The rights and preferences of each class of stock were the same both before and after the Reorganization. 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 rations for each series or the Company’s Preferred Stock. Upon the closing of the Company’s IPO in November 2017, all of the then outstanding convertible preferred shares automatically converted into shares of common stock.

In March 2017, the Company issued and sold 24,326,470 Class C preferred units at a price of $1.7749 per unit for proceeds of $43.0 million, net of issuance costs of $0.2 million. The sale of Class C preferred units met the definition of a qualified financing under the 2016 bridge unit agreements.

The Company issued 5,321,112 Class C preferred units upon the conversion of the 2016 bridge units in the amount of $8.5 million, at a conversion price of $1.60 per unit, which represented a discount of 10% to the price per unit paid by other investors in the Class C preferred unit financing. The conversion was accounted for as an extinguishment for accounting purposes. Accordingly, the Company recorded the Class C preferred units issued upon conversion of the 2016 bridge units at their aggregate fair value of $9.4 million and recorded a corresponding adjustment to extinguish the then-current carrying value of the 2016 bridge units of $8.5 million and the then-current fair value of the derivative liability related to the contingent prepayment option associated with the 2016 bridge units of $0.9 million. There was no gain or loss recognized upon the extinguishment.

In July 2017 the Company sold to its Chief Financial Officer 61,880 shares of the Company’s Series C preferred stock at a price of $1.7749 per share, for proceeds of $0.1 million.