Stock-Based Compensation Expense Included in Statement of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 6,829	$ 6,341	$ 7,534
Net share based employee compensation expense	6,829	6,341	7,534
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	2,098	1,461	1,683
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 4,731	$ 4,880	$ 5,851