Restructuring Plan - Additional Information (Detail) $ in Millions		1 Months Ended	12 Months Ended
	Oct. 06, 2017 Employee	Oct. 31, 2017 shares	Dec. 31, 2017 USD ($) shares
Restructuring Plan [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring plan effective date		Oct. 20, 2017
Number of employees reduced | Employee	45
Approximate reduction as a percent of workforce	62.00%
Restructuring plan notification date to affected employees		Oct. 06, 2017
One-time severance-related charge			$ 3.4
Accrued cash retention bonus			0.8
Restructuring Plan [Member] | General and administrative [Member]
Restructuring Cost And Reserve [Line Items]
One-time severance-related charge			0.7
Restructuring Plan [Member] | Research and development [Member]
Restructuring Cost And Reserve [Line Items]
One-time severance-related charge			$ 2.7
Severance Benefit Plan [Member]
Restructuring Cost And Reserve [Line Items]
Vesting period of shares granted to retained employees		2 years	2 years
Cash retention bonus percentage of each retained employees then current base salary		50.00%	50.00%
Required additional continuing employment period for retained employees		12 months	12 months
Severance Benefit Plan [Member] | Restricted stock units [Member]
Restructuring Cost And Reserve [Line Items]
Approximate number of shares granted to retained employees | shares		907,000	907,000