Income Taxes - Components of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Components Of Deferred Tax Assets [Abstract]
Capitalized research and development expenses	$ 53,557	$ 50,808
Net operating loss carryforwards	123,672	115,869
Research and development and other credit carryforwards	25,297	24,268
Other	15,400	18,748
Total deferred tax assets	217,926	209,693
Valuation allowance	$ (217,926)	$ (209,693)