Debt (Details) - USD ($) $ in Thousands		3 Months Ended		43 Months Ended
	Aug. 08, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Aug. 31, 2017	Aug. 14, 2017	Sep. 30, 2015
Debt Instrument [Line Items]
Interest paid		$ 2,200	$ 1,900
Common stock, shares issued		63,340,000		63,340,000	62,083,000
Amortization of debt discount		$ 98	98
Debt discount		500		$ 500
Interest expense		2,724	2,389
Net carrying amount		73,431		73,431	$ 73,357
Line Of Credit-UBS
Debt Instrument [Line Items]
Maximum borrowing capacity								$ 50,000
Outstanding balance		50,200		50,200
Accrued interest				1,200
Interest expense		$ 400	$ 300
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					$ 50,000
Additional interest in the event of default (as a percent)	3.00%
Debt instrument, term	84 months
Common stock, shares issued						300,000	300,000
Debt instrument, fee amount	$ 2,400
Payments of debt issuance costs	300
Debt discount	$ 2,700					$ 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
Unamortized debt discount		$ 1,600		$ 1,600