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Non-Controlling Interests - Spero Potentiator - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2017 | Aug. 31, 2016 | Jan. 31, 2016 | Nov. 30, 2015 | Feb. 28, 2015 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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 Potentiator, Inc. [Member] |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |
Common stock shares issued |  | 2,160 | 2,160 | 2,736 |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  |  | 49.90%
Proceeds from the issuance of common stock, net of issuance costs |  | $ 0 | $ 0 | $ 0 |  |  |  |  |  |
Non-controlling interest |  |  |  |  |  | $ 0 |  | $ 0 | $ (5,470,000) |
Spero Potentiator, Inc. [Member] | Anti- dilution Rights [Member] | Research and Development Expenses [Member] |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |
Fair value of derivative liability |  |  |  |  | $ 2,400,000 |  |  |  |  |
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |
Common stock shares issued |  | 2,160 | 2,160 | 2,736 | 996 |  |  |  |  |
Common stock issued, aggregate fair value |  |  |  |  | $ 1,100,000 |  |  |  |  |
Increase in non-controlling interest and reduction of derivative liability |  | $ 1,000,000 | $ 1,000,000 | $ 1,500,000 |  |  |  |  |  |
Cash payment for repurchase of shares | $ 1,000,000 |  |  |  |  |  |  |  |  |
Contingent consideration | 100,000 |  |  |  |  |  |  |  |  |
Milestone payments | 7,000,000 |  |  |  |  |  |  |  |  |
Cash payment for repurchase of shares | 2,500,000 |  |  |  |  |  |  |  |  |
Non-controlling interest | 6,400,000 |  |  |  |  |  |  |  |  |
Increase to accumulated deficit | $ 1,000,000 |  |  |  |  |  |  |  |  |
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Anti- dilution Rights [Member] |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 49.90% |  |  |  |  |
Proceeds from the issuance of common stock, net of issuance costs |  | $ 0 | $ 0 | $ 0 | $ 0 |  |  |  |  |
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Anti- dilution Rights [Member] | Prior to IND [Member] |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum amounts of investments for anti-dilution protection |  |  |  |  | 5,000,000 |  |  |  |  |
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Anti- dilution Rights [Member] | Post IND [Member] |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum amounts of investments for anti-dilution protection |  |  |  |  | 6,500,000 |  |  |  |  |
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Licensing Agreements [Member] |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |
Research and development expense |  |  |  |  | $ 1,100,000 |  |  |  |  |

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