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