Restructuring - Additional Information (Detail) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) Employee
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 9,155
Payments for restructuring	$ 3,300	$ 3,340
Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated | Employee		30
Number of positions eliminated, period percent		53.00%
Research and Development Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 7,900
General and Administrative Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		900
Miscellaneous Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 200