Summary of Significant Accounting Policies - Basis of Presentation and Liquidity (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 26, 2019	Mar. 31, 2019	Mar. 31, 2018	Apr. 30, 2019	Apr. 23, 2019	Dec. 31, 2018	Aug. 31, 2017	Aug. 14, 2017
Policies
Net (loss) income		$ (34,091)	$ (32,873)
Accumulated deficit		608,435				$ 574,529
Cash and cash equivalents		34,281				46,407
Marketable securities		93,805				107,461
Short-term Credit Line, outstanding balance		50,154				50,153
Long-term Term Loan, carrying amount		$ 73,431				$ 73,357
Common stock, shares issued		63,340,000				62,083,000
2017 Term Loan
Policies
Common stock, shares issued							300,000	300,000
Subsequent Event
Policies
Common stock, shares issued				25,000
Proceeds from issuance of common stock	$ 108,100
Payment of offering costs	$ 600
Equity offering | Subsequent Event
Policies
Common stock, shares issued					5,263,158
Stock issued (in dollars per share)					$ 19
Option for additional shares | Subsequent Event
Policies
Common stock, shares issued	789,473
Accumulated Deficit
Policies
Net (loss) income		$ (34,091)	$ (32,873)
Cumulative-effect adjustment upon adoption of ASU 2018-07		$ (185)