Subsequent Events	12 Months Ended
Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	19. Subsequent Events On February 11, 2020, the Company announced a rights offering pursuant to which it distributed to holders of its common stock and Series A Preferred Stock and Series B Preferred Stock, at no charge, non-transferable subscription rights to purchase shares of Spero common stock and Series C Convertible Preferred Stock (“Series C Preferred Stock”), with an aggregate offering value of $30.0 million. For each share of common stock (including shares of common stock issuable upon conversion of the Company’s outstanding shares of Series A Preferred Stock and Series B Preferred Stock) owned by holders of record as of 5:00 p.m., New York time, on February 10, 2020, such holders received 0.152 rights to purchase shares of Spero common stock (subject to the aggregate offering threshold and certain ownership limitations). Each whole right allowed holders to subscribe for one share of common stock at the subscription price equal to $9.00 per whole share (or an equivalent number of shares of Series C Preferred Stock). The total number of subscription rights issued to each stockholder was rounded down to the nearest whole number. Any participant in the rights offering that, following exercise of such participant’s subscription right, would be or become a holder of greater than 9.99% of the outstanding number of shares of the Company’s common stock following the offering may elect to instead purchase Series C Preferred Stock at a purchase price of $9,000 per share (ratably adjusted for fractional shares), and any such holder so electing would have a right to purchase one one-thousandth of a share of Series C Preferred Stock for each share of common stock it had a right to purchase under the subscription rights. Each share of Series C Preferred Stock is convertible into 1,000 shares of Spero common stock at the election of the holder, subject to beneficial ownership conversion limits applicable to the Series C Preferred Stock. The Series C Preferred Stock generally have no voting rights, except as required by law, and participate pari passu (on an as-converted basis) with any distribution of proceeds to holders of common stock and Series A Preferred Stock and Series B Preferred Stock, in the event of the Company’s liquidation, dissolution or winding up or the payment of a dividend on the common stock. At the closing of the rights offering on March 5, 2020, a total of 1,046,249 shares of the Company’s common stock and 2,287 shares of Series C Preferred Stock were issued for aggregate gross proceeds of $30.0 million.

Subsequent Events

12 Months Ended

Dec. 31, 2019

19. Subsequent Events

On February 11, 2020, the Company announced a rights offering pursuant to which it distributed to holders of its common stock and Series A Preferred Stock and Series B Preferred Stock, at no charge, non-transferable subscription rights to purchase shares of Spero common stock and Series C Convertible Preferred Stock (“Series C Preferred Stock”), with an aggregate offering value of $30.0 million. For each share of common stock (including shares of common stock issuable upon conversion of the Company’s outstanding shares of Series A Preferred Stock and Series B Preferred Stock) owned by holders of record as of 5:00 p.m., New York time, on February 10, 2020, such holders received 0.152 rights to purchase shares of Spero common stock (subject to the aggregate offering threshold and certain ownership limitations). Each whole right allowed holders to subscribe for one share of common stock at the subscription price equal to $9.00 per whole share (or an equivalent number of shares of Series C Preferred Stock). The total number of subscription rights issued to each stockholder was rounded down to the nearest whole number.

Any participant in the rights offering that, following exercise of such participant’s subscription right, would be or become a holder of greater than 9.99% of the outstanding number of shares of the Company’s common stock following the offering may elect to instead purchase Series C Preferred Stock at a purchase price of $9,000 per share (ratably adjusted for fractional shares), and any such holder so electing would have a right to purchase one one-thousandth of a share of Series C Preferred Stock for each share of common stock it had a right to purchase under the subscription rights. Each share of Series C Preferred Stock is convertible into 1,000 shares of Spero common stock at the election of the holder, subject to beneficial ownership conversion limits applicable to the Series C Preferred Stock. The Series C Preferred Stock generally have no voting rights, except as required by law, and participate pari passu (on an as-converted basis) with any distribution of proceeds to holders of common stock and Series A Preferred Stock and Series B Preferred Stock, in the event of the Company’s liquidation, dissolution or winding up or the payment of a dividend on the common stock.

At the closing of the rights offering on March 5, 2020, a total of 1,046,249 shares of the Company’s common stock and 2,287 shares of Series C Preferred Stock were issued for aggregate gross proceeds of $30.0 million.