Organization, Basis of Presentation and Going Concern - Narrative (Details) - USD ($) $ in Thousands		9 Months Ended
	Oct. 05, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Accumulated deficit		$ (532,332)		$ (445,223)
Document Period End Date		Sep. 30, 2017
Cash and cash equivalents		$ 64,730	$ 109,866	$ 91,551	$ 155,349
Net proceeds from issuance of common stock		$ 19,378	$ 0
Subsequent Event
Class of Stock [Line Items]
Net proceeds from issuance of common stock	$ 271,300