Restructuring - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Employee Employees
Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 9,390
Payments for restructuring	$ 5,756
Number of positions eliminated | Employee	30
Number of positions eliminated, period percent	53.00%
Restructuring Plan, July 2018 [Member] | One-time Employee Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 1,881
Payments for restructuring	$ 1,881
Restructuring Plan, December 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated | Employees	16
Restructuring Plan, December 2018 [Member] | One-time Employee Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 1,000
Research and Development Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	9,000
General and Administrative Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1,100
Miscellaneous Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 300