Restructuring Activities - Additional Information (Detail) $ in Thousands			3 Months Ended	12 Months Ended
	Nov. 07, 2018 Employee	Jan. 08, 2017 Employee	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Restructuring Cost And Reserve [Line Items]
Restructuring activity, announcement date				Nov. 07, 2018
Restructuring expenses				$ 1,289	$ 9,521
Number of employees after headcount reduction | Employee	27
Payments for restructuring			$ 400	996	9,667
One-time Termination Benefits
Restructuring Cost And Reserve [Line Items]
Restructuring expenses				1,300
One-time Termination Benefits | Research and Development Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses				1,000
One-time Termination Benefits | General and Administrative Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses				$ 300
January 2017 Corporate Restructuring [Member]
Restructuring Cost And Reserve [Line Items]
Reduction in headcount, percent		30.00%
Restructuring activity, announcement date				Jan. 08, 2017
Restructuring and related cost, number of expected employees after headcount reduction | Employee		80
January 2017 Corporate Restructuring [Member] | Discontinued Operations [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses				$ 9,500
January 2017 Corporate Restructuring [Member] | Contractual Termination Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses					$ 9,500
January 2017 Corporate Restructuring [Member] | One-time Termination Benefits
Restructuring Cost And Reserve [Line Items]
Severance payments payment period				1 year