Summary of Significant Accounting Policies - Basis of Presentation and Liquidity (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Apr. 26, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 30, 2019	Apr. 23, 2019	Dec. 31, 2018	Aug. 31, 2017
Policies
Net (loss) income		$ (32,416)	$ (33,824)	$ (66,507)	$ (66,697)
Accumulated deficit		640,851		640,851				$ 574,529
Cash and cash equivalents		29,928		29,928				46,407
Marketable securities		208,053		208,053				107,461
Short-term Credit Line, outstanding balance		50,146		50,146				50,153
Long-term Term Loan, carrying amount		$ 73,511		$ 73,511				$ 73,357
Common stock, shares issued		70,035,000		70,035,000				62,083,000
Proceeds from issuance of common stock	$ 108,100
Payment of offering costs	$ 600
2017 Term Loan
Policies
Common stock, shares issued						25,000			300,000
Equity offering
Policies
Common stock, shares issued							5,263,158
Stock issued (in dollars per share)							$ 19
Option for additional shares
Policies
Common stock, shares issued	789,473
Accumulated Deficit
Policies
Net (loss) income		$ (32,416)	$ (33,824)	$ (66,507)	$ (66,697)
Cumulative-effect adjustment upon adoption of ASU 2018-07				$ (185)