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Australia Research and Development Tax Incentive - Additional Information (Detail) - Spero Potentiator Australia Pty Limited [Member] - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Australia [Member] |  |
Schedule Of Income Tax [Line Items] |  |
Tax incentive receivable | $ 1.9 | $ 0.1
Research and Development Expenses [Member] |  |
Schedule Of Income Tax [Line Items] |  |
Percentage of refundable tax incentive | 43.50% |
Reduction to research and development expenses | $ 1.8 | $ 0.1

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