Income Taxes - Schedule of Temporary Differences That Give Rise to Significant Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets
Net operating losses	$ 61,905	$ 61,189
Capitalized research and development expenses	423	564
Credit carryforwards	167,501	166,459
Deferred compensation	1,890	1,479
Election out of installment method	820	820
Accrued expenses	124
Other temporary differences	720	1,089
Installment sale tax basis	15,869	15,868
Total gross deferred tax assets	249,252	247,468
Valuation allowance	$ (249,252)	$ (247,468)	$ (248,478)