Restructuring Activities- Additional Information (Detail) $ in Thousands		3 Months Ended			9 Months Ended		12 Months Ended
	Jul. 12, 2019 Employee	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Restructuring Cost And Reserve [Line Items]
Restructuring expenses				$ 0	$ 4,825	$ 0
Payments for restructuring		$ 800	$ 400	$ 200	5,624	$ 600	$ 900
Number of employees | Employee	0
One-time Termination Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses		100			4,800		1,300
One-time Termination Benefits [Member] | Research and Development Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses					2,000		1,000
One-time Termination Benefits [Member] | General and Administrative Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses		$ 100			$ 2,800		$ 300
November 2018 Corporate Restructuring [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring activity, announcement date					Nov. 07, 2018