Organization, Basis of Presentation and Going Concern - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 489,538	$ 445,223
Cash and cash equivalents	$ 65,761	$ 91,551	$ 127,797	$ 155,349