Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred Tax Assets:
NOL carryforwards	$ 86,208	$ 78,289
Research and development tax credit carryforwards	13,060	12,212
Depreciation and amortization	12,262	3,591
Capitalized research and development expenditures	29,121	24,059
Impairment of investments	120	120
Stock options	4,419	3,534
Accrued expenses and other	113	114
Total Gross Deferred Tax Asset	145,303	121,919
Valuation Allowance	(145,303)	(121,919)
Net Deferred Tax Asset	$ 0	$ 0