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Non-Controlling Interests - Spero Europe - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jan. 31, 2016 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | May 31, 2017
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |
Research and development expense |  | $ 8,925,000 | $ 5,999,000 | $ 32,869,000 | $ 26,333,000 | $ 11,125,000 |
Non-controlling interest |  | 355,000 |  | 355,000 | $ (3,808,000) |  |
Spero Europe, Ltd. [Member] |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 5.00% |  |
Termination of agreement notice period | 30 days |  |  |  |  |  |
Non-controlling interest |  | $ 0.0 |  | $ 0 | $ (21,000) |  |
Spero Europe, Ltd. [Member] | Maximum [Member] |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |
Non-controlling interest |  |  |  |  |  |  | $ 100,000
Spero Europe, Ltd. [Member] | Licensing Agreements [Member] | Anti- dilution Rights [Member] |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |
Research and development expense | $ 200,000 |  |  |  |  |  |
Spero Europe, Ltd. [Member] | Promiliad Biopharma, Inc. [Member] |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |
Ownership percentage | 5.00% |  |  |  |  |  |
Common stock issued, aggregate fair value | $ 100,000 |  |  |  |  |  |
Milestone payments | 4,100,000 |  |  |  |  |  |
Proceeds from the issuance of common stock, net of issuance costs | 0 |  |  |  |  |  |
Maximum amounts of investments for anti-dilution protection | $ 5,000,000 |  |  |  |  |  |
Spero Europe, Ltd. [Member] | Promiliad Biopharma, Inc. [Member] | Anti- dilution Rights [Member] |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |
Ownership percentage | 5.00% |  |  |  |  |  |
Spero Europe, Ltd. [Member] | Promiliad Biopharma, Inc. [Member] | Licensing Agreements [Member] |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |
Research and development expense | $ 100,000 |  |  |  |  |  |

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