Convertible Notes - Estimated Fair Value (Details) - Compounded Bifurcated Derivatives - Binomial Lattice Model - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Calculated aggregate value	$ 1,474	$ 3,136
Closing price per share of common stock (usd per share)	$ 1.50	$ 3.20
Contractual remaining term	7 years 8 months	8 years 8 months
Contractual interest rate	4.00%	4.00%
Volume-weighted average conversion rate
Debt Instrument [Line Items]
Measurement input	17.04667	17.04667
Risk-free interest rate (term structure) | Minimum
Debt Instrument [Line Items]
Measurement input	0.0244	0.0128
Risk-free interest rate (term structure) | Maximum
Debt Instrument [Line Items]
Measurement input	0.0269	0.0240
Dividend yield
Debt Instrument [Line Items]
Measurement input	0	0
Credit Spread
Debt Instrument [Line Items]
Measurement input	0.3177	0.3698
Volatility
Debt Instrument [Line Items]
Measurement input	0.875	0.990