Restructuring Plan - Additional Information (Detail) - Restructuring Plan [Member] $ in Millions		1 Months Ended	3 Months Ended	6 Months Ended	9 Months Ended
	Jun. 26, 2018 Employee	Oct. 31, 2017	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Restructuring Cost And Reserve [Line Items]
Restructuring plan effective date		Oct. 20, 2017
Total number of employees reduced | Employee	14
Approximate reduction as a percent of workforce	59.00%
Restructuring plan notification date to affected employees	Jun. 26, 2018
One-time severance-related charge			$ 0.2	$ 3.5	$ 3.7
Payment of one-time severance-related charge					3.7
General and administrative [Member]
Restructuring Cost And Reserve [Line Items]
One-time severance-related charge					1.2
Research and development [Member]
Restructuring Cost And Reserve [Line Items]
One-time severance-related charge					$ 2.5