Nature of Business and Plan of Operations (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Nature of Business and Plan of Operations
Net loss	$ (80,237)	$ (26,212)
Accumulated deficit	(1,094,729)		$ (1,028,582)
Product revenue	0
Cash and cash equivalents	$ 345,058	$ 150,337	$ 267,107	$ 159,964