Summary of Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
	Jul. 16, 2018	Aug. 08, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jul. 12, 2018	Dec. 31, 2017	Aug. 14, 2017	Dec. 31, 2016
Policies
Net (loss) income			$ (29,616)	$ (27,865)	$ (96,313)	$ (90,411)
Accumulated deficit			542,688		542,688			$ 446,375
Cash and cash equivalents			34,535	$ 11,211	34,535	$ 11,211		12,620		$ 15,256
Marketable securities			130,891		130,891			106,247
Short-term Credit Line, outstanding balance			50,135		50,135			50,112
Long-term Term Loan, carrying amount			$ 73,284		$ 73,284			$ 73,065
Common stock, shares issued			61,673,000		61,673,000			54,040,000
Restricted cash
Restricted cash, current portion			$ 4,222		$ 4,222			$ 59
Restricted cash, long term portion			$ 342		$ 342			$ 342
July 2018 Offering
Policies
Common stock, shares issued							4,500,000
Stock price							$ 20
Option for additional shares
Policies
Common stock, shares issued							675,000
July 2018 Offering inclusive of option for additional shares
Policies
Proceeds from issuance of common stock	$ 97,300
Payment of offering costs	$ 500
2017 Term Loan
Policies
Maximum borrowing capacity		$ 100,000
Available borrowing capacity		75,000
Remaining borrowing capacity		$ 25,000
Debt instrument, term		84 months
Common stock, shares issued									300,000
Stock price		$ 8.16
2017 Term Loan | Base Rate
Policies
Spread on interest rate (as a percent)		8.75%
Base interest rate (as a percent)		1.00%