Share-Based Payments - Inducement Awards (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-Based Payments
Share-based compensation expense		$ 5,154	$ 5,732	$ 2,545
Inducement Awards
Share-Based Payments
Share-based compensation expense		$ 200
Fair Value Assumptions
Expected volatility (as a percent)		59.80%
Expected term of options (in years)		6 years 1 month 6 days
Risk-free interest rate (as a percent)		2.90%
Dividend yield (as a percent)		0.00%
Additional disclosures
Weighted-average remaining contractual life		9 years 7 months 6 days
Inducement Awards | Vesting period, year one
Share-Based Payments
Percentage that vests during the period	25.00%
Inducement Awards | Vesting period, years 2-4
Share-Based Payments
Percentage that vests during the period	75.00%
Monthly vesting period	36 months
Inducement Awards | Non-statutory option
Share-Based Payments
Options exercise price (in dollars per share)	$ 3.53
Exercise period	10 years
Vesting period	4 years
Options weighted average grant date fair value (in dollars per share)		$ 2.05
Inducement Awards | Non-statutory option | Chief Executive Officer
Share-Based Payments
Options granted during the period	850,000
Inducement Awards | Restricted Stock Units ("RSUs")
Share-Based Payments
Restricted stock weighted average grant date fair value (in dollars per share)		$ 3.53
Inducement Awards | Restricted Stock Units ("RSUs") | Chief Executive Officer
Share-Based Payments
Restricted stock granted during the period	150,000
Inducement Awards | Restricted Stock Units ("RSUs") | Upon receipt of FDA approval
Share-Based Payments
Percentage that vests during the period	50.00%
Inducement Awards | Restricted Stock Units ("RSUs") | First anniversary of FDA approval
Share-Based Payments
Percentage that vests during the period	50.00%