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Non-Controlling Interests - Spero Gyrase - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2016 | 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
Spero Gyrase, Inc. [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Ownership percentage |  |  |  |  | 20.00% |
Spero Gyrase, Inc. [Member] | Anti- dilution Rights [Member] | Research and Development Expenses [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Fair value of derivative liability | $ 1,600,000 |  |  |  |  |
Spero Gyrase, Inc. [Member] | Aviragen Therapeutics, Inc. [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Common stock shares issued | 200 |  |  |  |  |
Common stock issued, aggregate fair value | $ 1,100,000 |  |  |  |  |
Ownership percentage | 20.00% |  |  |  |  |
Spero Gyrase, Inc. [Member] | Aviragen Therapeutics, Inc. [Member] | Anti- dilution Rights [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Ownership percentage | 20.00% |  |  |  |  |
Proceeds from the issuance of common stock, net of issuance costs | $ 0 |  |  |  |  |
Spero Gyrase, Inc. [Member] | Aviragen Therapeutics, Inc. [Member] | Anti- dilution Rights [Member] | Maximum [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Gross proceeds from equity financings | 8,000,000 |  |  |  |  |
Spero Gyrase, Inc. [Member] | Aviragen Therapeutics, Inc. [Member] | Technology-Based Intangible Assets [Member] |  |  |  |  |  |
Noncontrolling Interest [Line Items] |  |  |  |  |  |
Research and development expense | $ 1,100,000 |  |  |  |  |

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