Nature of the Business and Basis of Presentation - Going Concern (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash	$ 217,452	$ 217,452	$ 204,290	$ 217,452	$ 204,290	$ 131,468	$ 23,692
Net losses		39,269	$ 38,440	124,731	57,193
Accumulated deficit	327,377	327,377		327,377		$ 202,646
Liability For Sale of Future Royalties [Line Items]
Proceeds from sale of common stock	49,889			55,000	$ 179,996
RPI Agreement
Liability For Sale of Future Royalties [Line Items]
Proceeds from royalty agreement	149,623
Issuance costs	377	$ 377		$ 377
RPI Funding Agreement
Liability For Sale of Future Royalties [Line Items]
Proceeds from royalty agreement	$ 100,000
RPI Purchase Agreement
Liability For Sale of Future Royalties [Line Items]
Common stock sold (shares)	1,111,111
Common stock sold, price per share (in dollars per share)	$ 45.00	$ 45.00		$ 45.00
Proceeds from sale of common stock	$ 50,000