Restructuring Activities - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost And Reserve [Line Items]
Accrued restructuring expenses		$ 7,000	$ 122,000	$ 7,000	$ 122,000	$ 66,000	$ 921,000
Restructuring expenses		0		0	4,825,000
Payments for restructuring			800,000		5,624,000
Maximum [Member]
Restructuring Cost And Reserve [Line Items]
Payments for restructuring		$ 100,000		$ 100,000
Scenario Forecast [Member] | Maximum [Member]
Restructuring Cost And Reserve [Line Items]
Payments for restructuring	$ 100,000
One-time Termination Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses			100,000		4,800,000
One-time Termination Benefits [Member] | Research and Development Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses					2,000,000.0
One-time Termination Benefits [Member] | General and Administrative Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses			$ 100,000		$ 2,800,000
November 2018 Corporate Restructuring [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring activity, announcement date				Nov. 07, 2018