Significant Component of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Net operating loss carryforwards	$ 147,004	$ 124,115
Deferred tax assets:
Start-up and organizational costs	25,909	30,480
Research and development credit carryforwards	37,183	35,130
Stock compensation	1,478	1,087
Capitalized acquisition costs	3,691	4,501
Lease liability	1,225	626
Depreciation	71	176
Other	135	1,186
Deferred Tax Assets Gross	216,696	197,301
Less valuation allowance	(215,513)	(196,696)
Total deferred tax assets	1,183	605
Deferred tax liabilities:
Right of use asset	(1,183)	(605)
Total deferred tax liabilities	(1,183)	(605)
Net deferred taxes	$ 0	$ 0