Australia Research and Development Tax Incentive - Additional Information (Detail) - Spero Potentiator Australia Pty Limited [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Australia [Member]
Schedule Of Income Tax [Line Items]
Tax incentive receivable	$ 0.7		$ 0.7		$ 1.9
Research and Development Expenses [Member]
Schedule Of Income Tax [Line Items]
Percentage of refundable tax incentive			43.50%
Reduction to research and development expenses	$ 0.3	$ 0.1	$ 0.7	$ 0.8