Nature of Business and Plan of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Nature of Business and Plan of Operations
Net loss	$ (38,613)	$ (17,345)
Accumulated deficit	(1,053,105)		$ (1,028,582)
Product revenue	0
Cash and cash equivalents	$ 218,383	$ 126,568	$ 267,107	$ 159,964