Significant Component of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 106,430	$ 89,098
Start-up and organizational costs	33,977	37,488
Research and development credit carryforwards	33,684	32,395
Stock compensation	990	1,330
Capitalized acquisition costs	5,160	5,822
Deferred revenue	0	11,126
Depreciation	132	136
Other	920	993
Deferred Tax Assets Gross	181,293	178,388
Less valuation allowance	(181,293)	(178,388)
Effective tax rate	$ 0	$ 0