Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands		3 Months Ended
	Aug. 08, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Aug. 14, 2017	Dec. 31, 2016
Policies
Common stock, shares issued		54,251,000		54,040,000
Net (loss) income		$ (32,873)	$ (33,480)
Accumulated deficit		479,248		$ 446,375
Cash and cash equivalents		33,674	$ 9,086	12,620		$ 15,256
Marketable securities		80,023		106,247
Short-term Credit Line, outstanding balance		50,125		50,112
Long-term Term Loan, carrying amount		73,138		73,065
Restricted cash
Restricted cash, current portion		5,674		59
Restricted cash, long term portion		$ 342		$ 342
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
2017 Term Loan | Base Rate
Policies
Spread on interest rate (as a percent)	8.75%
Base interest rate (as a percent)	1.00%