Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 507,235	$ 712,124
Research and development credits	83,461	77,998
Capitalized research	1,016	5,117
Milestone Rights	1,908	3,242
Accrued expenses	211	440
Loss on purchase commitment	23,654	36,775
Non-qualified stock option expense	7,004	17,331
Capitalized patent costs	5,194	8,781
Other	795	7,380
Depreciation	23,820	45,310
Total net deferred tax assets	654,298	914,498
Valuation allowance	(654,298)	(914,498)
Net deferred tax assets	$ 0	$ 0