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Common Stock - Additional Information (Detail) |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Oct. 20, 2017 | Jun. 30, 2017 $ / shares shares | Dec. 31, 2017 $ / shares shares | Dec. 31, 2016 $ / shares shares | Nov. 14, 2017 $ / shares | Nov. 06, 2017 $ / shares shares | Dec. 31, 2015 shares
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Common stock, shares authorized |  | 61,917,986 | 60,000,000 | 0 |  |  |
Common stock, par value | $ / shares |  | $ 0.001 | $ 0.001 | $ 0.001 |  |  |
Common stock, voting rights |  |  | Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. |  |  |  |
Number of shares repurchased unvested common units upon forfeiture |  |  |  | 21,116 |  |  |
Stock price of shares repurchased per unit | $ / shares |  |  |  | $ 0.001 |  |  |
Unvested restricted common units outstanding |  |  | 0 | 7,062 |  |  | 75,210
Common Stock [Member] |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Reverse stock split ratio | 0.164544 |  |  |  |  |  |
Reverse stock split | one-for-6.0774 |  |  |  |  |  |
Spero Therapeutics, LLC [Member] |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Conversion of stock |  | 1 |  |  |  |  |
Initial Public Offering [Member] |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Common stock, shares authorized |  |  |  |  |  | 60,000,000 |
Common stock, par value | $ / shares |  |  |  |  |  | $ 0.001 |
Initial Public Offering [Member] | Common Stock [Member] |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |
Stock price of shares repurchased per unit | $ / shares |  |  |  |  | $ 14.00 | $ 14.00 |

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