Basis of Presentation - (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basis of Presentation
Cash, cash equivalents and marketable securities	$ 105,600		$ 105,600
Net Loss	$ (7,243)	$ (26,363)	(141,781)	$ (89,190)
Net cash used in operations			$ 59,805	$ 80,410