Summary of Significant Accounting Policies - Basis of Presentation and Liquidity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
	Oct. 17, 2019	Apr. 26, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Apr. 30, 2019	Apr. 23, 2019	Dec. 31, 2018	Aug. 31, 2017
Policies
Net (loss) income			$ (23,137)	$ (29,616)	$ (89,644)	$ (96,313)
Accumulated deficit			663,988		663,988				$ 574,529
Cash and cash equivalents			51,609		51,609				46,407
Marketable securities			186,700		186,700				107,461
Short-term Credit Line, outstanding balance			50,131		50,131				50,153
Long-term Term Loan, carrying amount			$ 73,583		$ 73,583				$ 73,357
Common stock, shares issued			70,850,000		70,850,000				62,083,000
Payment of offering costs		$ 600
Proceeds from issuance of common stock		$ 108,100
2017 Term Loan
Policies
Common stock, shares issued							25,000			300,000
Subsequent Event
Policies
Payment of offering costs	$ 400
Proceeds from issuance of common stock	$ 216,200
Equity offering
Policies
Common stock, shares issued								5,263,158
Stock issued (in dollars per share)								$ 19
Equity offering | Subsequent Event
Policies
Common stock, shares issued	5,714,286
Stock issued (in dollars per share)	$ 35
Option for additional shares
Policies
Common stock, shares issued		789,473
Option for additional shares | Subsequent Event
Policies
Common stock, shares issued	857,142
Accumulated Deficit
Policies
Net (loss) income			$ (23,137)	$ (29,616)	$ (89,644)	$ (96,313)
Cumulative-effect adjustment upon adoption of ASU 2018-07					$ (185)