Warrants - Aggregate Fair Values, Inputs and Assumptions (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares
Warrant
Fair value assumptions
Calculated aggregate value | $	$ 144	$ 152
Weighted average exercise price per share (usd per share)	$ 22.50	$ 22.50
Closing price per share of common stock (usd per share)	$ 1.90	$ 1.50
Volatility | Warrant
Fair value assumptions
Warrant liability, measurement input	0.928	0.941
Weighted average remaining expected life | Warrant
Fair value assumptions
Weighted average remaining expected life	2 years 3 months	2 years 8 months
Risk-free interest rate | Warrant
Fair value assumptions
Warrant liability, measurement input	0.0174	0.0245
Dividend yield
Fair value assumptions
Warrant liability, measurement input	0
Dividend yield | Warrant
Fair value assumptions
Warrant liability, measurement input	0	0