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Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Aug. 31, 2016 | Feb. 29, 2016 | Jan. 31, 2016 | Nov. 30, 2015 | Jul. 01, 2015 | Jun. 30, 2015 | Feb. 28, 2015 | Apr. 30, 2014 | Mar. 31, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Mar. 31, 2016 | Jan. 31, 2015 | Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value assets level 1 to level 2 transfers |  |  | $ 0 |  |  |  |  |  |  |  |  | $ 0 |  | $ 0 | $ 0 |  |  |
Fair value assets level 2 to level 1 transfers |  |  | 0 |  |  |  |  |  |  |  |  | 0 |  | 0 | 0 |  |  |
Fair value liabilities level 1 to level 2 transfers |  |  | 0 |  |  |  |  |  |  |  |  | 0 |  | 0 | 0 |  |  |
Fair value liabilities level 2 to level 1 transfers |  |  | 0 |  |  |  |  |  |  |  |  | 0 |  | 0 | 0 |  |  |
Transfer of financial asset into level 3 of fair value |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 | 0 |  |  |
Transfer of financial liabilities into level 3 of fair value |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 | 0 |  |  |
Roche Collaboration Agreements [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of issuance price |  |  |  |  |  |  |  |  |  |  | 90.00% |  |  |  |  |  |  |
Research and development agreement, potential investment |  |  |  |  |  |  |  |  |  |  | $ 2,000,000 |  |  |  |  |  |  |
Class A Tranche Rights[Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities |  |  |  |  |  |  |  |  | $ 2,400,000 |  |  |  |  |  |  |  |  |
Derivative liability |  |  |  |  | $ (2,400,000) |  |  |  |  |  |  |  |  |  |  |  |  |
Class B Tranche Rights [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities |  |  | 300,000 |  | $ 900,000 |  |  |  |  |  |  |  |  |  | 300,000 |  |  |
Derivative liability |  |  | (600,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
January 2015 Bridge Units [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liability |  |  |  |  |  |  |  | $ (1,400,000) |  |  |  |  |  |  |  |  |  |
Class A Preferred Unit [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred units discount percentage |  |  |  |  |  |  |  | 10.00% | 20.00% |  |  |  |  |  |  |  |  |
Conversion price of redeemable preferred share |  |  |  |  |  |  |  | $ 3.51 | $ 3.90 |  |  |  |  |  |  |  |  |
Contingent prepayment option [Member] | January 2015 Bridge Units [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,300,000 |
Contingent prepayment option [Member] | December 2016 Bridge Units [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities |  |  | 900,000 |  |  |  |  |  |  |  |  |  |  |  | 900,000 |  |  |
Contingent prepayment option [Member] | December 2016 Bridge Units [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liability |  | $ 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities |  |  |  |  |  |  |  |  | $ 100,000 |  |  |  |  |  |  |  |  | $ 200,000
Derivative liability |  |  |  |  |  |  |  |  | $ (100,000) |  |  |  |  |  |  |  |  |
Spero Gyrase, Inc. [Member] | Anti- dilution Rights [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities | $ 200,000 |  | $ 200,000 |  |  |  |  |  |  |  |  | $ 200,000 | $ 200,000 | $ 200,000 | $ 200,000 | $ 1,600,000 |  |
Derivative liability | $ (1,400,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value inputs probability of funding |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.00% | 100.00% |  |  |
Decrease in derivative liability |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,400,000 |  |  |  |  |
Spero Potentiator, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued |  |  |  | 2,160 |  | 2,160 | 2,736 |  |  |  |  |  |  |  |  |  |  |
Fair value inputs probability of funding |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued |  |  |  | 2,160 |  | 2,160 | 2,736 |  |  | 996 |  |  |  |  |  |  |  |
Amount of shares issued |  |  |  | $ 1,000,000 |  | $ 1,000,000 | $ 1,500,000 |  |  |  |  |  |  |  |  |  |  |
Spero Potentiator, Inc. [Member] | Anti- dilution Rights [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities |  |  |  |  |  |  |  |  |  | $ 2,400,000 |  |  |  |  |  |  |  |
Spero Europe, Ltd. [Member] | Anti- dilution Rights [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |
Derivative liability |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000 |  |  |  |
Fair value inputs probability of funding |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.00% | 100.00% |  |  |

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