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Non-Controlling Interests - Spero Cantab - Additional Information (Detail) | 1 Months Ended | 12 Months Ended
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Dec. 31, 2017 USD ($) Option | Jul. 31, 2017 USD ($) | Jun. 30, 2016 USD ($) Option shares | Dec. 31, 2017 USD ($) Option | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Research and development expense |  |  |  | $ 32,869,000 | $ 26,333,000 | $ 11,125,000
Non-controlling interest | $ 355,000 |  |  | 355,000 | $ (3,808,000) |
Spero Cantab [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Ownership percentage |  |  |  |  | 12.50% |
Non-controlling interest | 0 |  |  | 0 | $ 1,303,000 |
Spero Cantab [Member] | Cantab Agreements [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Common stock shares issued | shares |  |  | 125 |  |  |
Ownership percentage |  |  | 12.50% |  |  |
Common stock issued, aggregate fair value |  |  | $ 1,600,000 |  |  |
Upfront consideration |  |  | 300,000 |  |  |
Non-controlling interest |  |  | 1,600,000 |  |  |
Cash payment for repurchase of shares |  | $ 200,000 |  |  |  |
Spero Cantab [Member] | Cantab Agreements [Member] | Contingent Milestone Payments [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Cash payment for repurchase of shares |  | $ 100,000 |  |  |  |
Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Research and development expense |  |  | 300,000 |  |  |
Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member] | Niaid [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Maximum potential funding from government contract |  |  | $ 5,700,000 |  |  |
Number of option period for funding from government contract | Option |  |  | 3 |  |  |
Maximum received fund | $ 1,300,000 |  |  | $ 1,300,000 |  |
Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member] | Niaid [Member] | First Option [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Number of option period for funding from government contract | Option | 2 |  |  | 2 |  |
Committed amount from government contract | $ 5,100,000 |  |  | $ 5,100,000 |  |
Spero Cantab [Member] | New Pharma License Holding Limited [Member] | Cantab Agreements [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Research and development expense |  |  | $ 1,600,000 |  |  |
Spero Cantab [Member] | Pro Bono Bio PLC [Member] | Cantab Agreements [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Maximum amounts of investments for anti-dilution protection |  |  | $ 8,000,000 |  |  |
Spero Cantab [Member] | Pro Bono Bio PLC [Member] | Cantab Agreements [Member] | Minimum [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Ownership percentage |  |  | 5.00% |  |  |
Spero Cantab [Member] | Pro Bono Bio PLC [Member] | Cantab Agreements [Member] | Maximum [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Ownership percentage |  |  | 12.50% |  |  |

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