Convertible Notes - Estimated Fair Values (Details) - Compounded Bifurcated Derivatives - Binomial Lattice Model - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Calculated aggregate value	$ 2,557	$ 1,474
Closing price per share of common stock (usd per share)	$ 1.90	$ 1.50
Contractual interest rate	4.00%	4.00%
Volume-weighted average conversion rate (usd per share)	$ 17.04667	$ 17.04667
Contractual remaining term
Debt Instrument [Line Items]
Contractual remaining term	7 years 2 months	7 years 8 months
Risk-free interest rate \| Minimum
Debt Instrument [Line Items]
Derivative liability, measurement input	0.0171	0.0244
Risk-free interest rate \| Maximum
Debt Instrument [Line Items]
Derivative liability, measurement input	0.0218	0.0269
Dividend yield
Debt Instrument [Line Items]
Derivative liability, measurement input	0	0
Credit Spread
Debt Instrument [Line Items]
Derivative liability, measurement input	0.2801	0.3177
Volatility
Debt Instrument [Line Items]
Derivative liability, measurement input	0.892	0.875

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- Definition

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Fair Value Assumptions, Volume-weighted Average Conversion Rate

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- Definition

Fixed interest rate related to the interest rate derivative.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Value of input used to measure derivative liability.

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