Summary of Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
	Jul. 16, 2018	Aug. 08, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jul. 12, 2018	Dec. 31, 2017	Aug. 14, 2017	Dec. 31, 2016
Policies
Net (loss) income			$ (33,824)	$ (29,066)	$ (66,697)	$ (62,546)
Accumulated deficit			513,072		513,072			$ 446,375
Cash and cash equivalents			12,306	$ 7,138	12,306	$ 7,138		12,620		$ 15,256
Marketable securities			70,901		70,901			106,247
Short-term Credit Line, outstanding balance			50,132		50,132			50,112
Long-term Term Loan, carrying amount			$ 73,211		$ 73,211			$ 73,065
Common stock, shares issued			55,454,000		55,454,000			54,040,000
Restricted cash
Restricted cash, current portion			$ 5,612		$ 5,612			$ 59
Restricted cash, long term portion			$ 342		$ 342			$ 342
Subsequent Event | July 2018 Offering
Policies
Common stock, shares issued							4,500,000
Stock price							$ 20
Subsequent Event | Option for additional shares
Policies
Common stock, shares issued							675,000
Subsequent Event | July 2018 Offering inclusive of option for additional shares
Policies
Proceeds from issuance of common stock	$ 96,600
Payment of offering costs	$ 700
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%