Basis of Presentation - (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basis of Presentation
Cash, cash equivalents and marketable securities	$ 140,500		$ 140,500
Net Loss	$ (26,363)	$ (29,598)	(89,190)	$ (96,223)
Net cash used in operations			$ 80,410	$ 92,687