Debt (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended		25 Months Ended
	Aug. 08, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2017	Aug. 14, 2017	Sep. 30, 2015
Debt Instrument [Line Items]
Common stock, shares issued		61,673,000		61,673,000			54,040,000
Interest expense		$ 2,781	$ 1,477	$ 7,730	$ 1,878
Interest expense		2,100		6,100
Amortization of debt discount		100		293	$ 51
Debt discount		600		600
Unamortized debt discount		6,100		6,100
Net carrying amount		73,284		73,284			$ 73,065
Line Of Credit-UBS
Debt Instrument [Line Items]
Maximum borrowing capacity									$ 50,000
Outstanding balance		50,100		50,100
Interest expense		400		1,100
Accrued interest				$ 1,100		$ 1,100
Line Of Credit-UBS | 30-day LIBOR
Debt Instrument [Line Items]
Spread on interest rate (as a percent)				1.10%
2017 Term Loan
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 100,000
Available borrowing capacity	75,000
Remaining borrowing capacity	$ 25,000
Additional interest in the event of default (as a percent)	3.00%
Debt instrument, term	84 months
Common stock, shares issued								300,000
Debt instrument, fee amount	$ 2,400
Stock price	$ 8.16
Payments of debt issuance costs	$ 300
Debt discount	$ 2,700
2017 Term Loan | Minimum
Debt Instrument [Line Items]
Prepayment premium (as a percent)	2.50%
2017 Term Loan | Maximum
Debt Instrument [Line Items]
Prepayment premium (as a percent)	12.50%
2017 Term Loan | Base Rate
Debt Instrument [Line Items]
Spread on interest rate (as a percent)	8.75%
Base interest rate (as a percent)	1.00%
2017 Term Loan | Direct reduction from outstanding debt balance
Debt Instrument [Line Items]
Debt instrument, fee amount	$ 2,000	$ 1,700		$ 1,700