Restructuring - Additional Information (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) Employees Employee
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%
Restructuring charges		$ 9,390
Payments for restructuring	$ 1,657	5,756
Revised estimates during the year/ period	(1,942)	235
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
Revised estimates during the year/ period		2
Restructuring Plan, July 2018 [Member] | Clinical Trial Closing [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		5,744
Payments for restructuring	1,579	2,204
Revised estimates during the year/ period	(1,954)	$ 41
Restructuring Plan, November 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated | Employees		16
Restructuring Plan, November 2018 [Member] | One-time Employee Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	660	$ 984
Payments for restructuring	702	$ 922
Research and Development Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring recoveries	1,900
General and Administrative Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 600