Significant Component of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 89,098	$ 100,790
Start-up and organizational costs	37,488	59,360
Research and development credit carryforwards	32,395	34,845
Stock compensation	1,330	2,014
Capitalized acquisition costs	5,822	9,389
Deferred revenue	11,126	18,636
Depreciation	136	227
Other	993	1,537
Deferred Tax Assets Gross	178,388	226,798
Less valuation allowance	(178,388)	(226,798)
Effective tax rate	$ 0	$ 0