Debt (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Aug. 08, 2017	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Aug. 14, 2017	Sep. 30, 2015
Debt Instrument [Line Items]
Common stock, shares issued			62,082,348	54,040,000
Payments of debt issuance costs				$ 287
Amortization of debt discount			$ 391	154
Interest expense			10,476	4,213	$ 533
Net carrying amount			73,357	73,065
Line Of Credit-UBS
Debt Instrument [Line Items]
Maximum borrowing capacity							$ 50,000
Interest expense			1,600	1,000	$ 500
Interest paid			1,500	500
Outstanding balance			50,200
Debt, interest accrued			$ 1,200
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		$ 75,000	75,000
Remaining borrowing capacity	$ 25,000	$ 50,000	50,000
Interest expense			8,600	3,200
Interest paid			6,100	3,100
Repayments			0
Additional interest in the event of default (as a percent)		3.00%
Common stock, shares issued		300,000				300,000
Debt instrument, fee amount		$ 2,400
Stock issued (in dollars per share)	$ 8.16
Accrued interest			2,100
Payments of debt issuance costs	$ 300
Amortization of debt discount			100
Debt discount		2,700	600
Debt, increase (decrease)		$ (2,000)
Less: unamortized debt discount			(1,643)	(1,935)
Net carrying amount			$ 73,357	$ 73,065
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%	8.50%
Spread on base interest rate (as a percent)			1.00%