Restructuring Plan - Additional Information (Detail)		1 Months Ended	12 Months Ended
	Oct. 06, 2017 Employee	Oct. 31, 2017 shares	Dec. 31, 2019 shares	Dec. 31, 2018 USD ($)
Restructuring Plan [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring plan effective date		Oct. 20, 2017
Total 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
Severance-related charges				$ 4,200,000
Balance amount owed under restructuring activities				0
Restructuring Plan [Member] | General and administrative [Member]
Restructuring Cost And Reserve [Line Items]
Severance-related charges				1,700,000
Restructuring Plan [Member] | Research and development [Member]
Restructuring Cost And Reserve [Line Items]
Severance-related charges				$ 2,500,000
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
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	151,000