Preferred Stock and Common Stock Warrants - Allocation of net cash proceeds (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Allocation of net cash proceeds
Gross proceeds	$ 79,055	$ 30,100
Equity issuance costs	(2,104)
Net proceeds - Initial Fair Value Allocation	76,951
BCF | Additional Paid-in Capital
Allocation of net cash proceeds
Allocation to BCF	2,530
Per balance sheet upon issuance	2,530
Series A Convertible Preferred stock
Allocation of net cash proceeds
Net proceeds - Initial Fair Value Allocation	41,079
Allocation to BCF	(2,530)
Per balance sheet upon issuance	$ 38,549
Contractual conversion price	$ 7.50
Series A Convertible Preferred stock | HGC
Allocation of net cash proceeds
Accounting conversion price	6.24
Series A Convertible Preferred stock | Fund Purchaser
Allocation of net cash proceeds
Accounting conversion price	$ 5.58
Series A Convertible Preferred stock | BCF
Allocation of net cash proceeds
Accretion of stock discount as a deemed dividend	$ 2,500
Series B Convertible Preferred stock
Allocation of net cash proceeds
Net proceeds - Initial Fair Value Allocation	26,159
Per balance sheet upon issuance	26,159
Common Stock | Additional Paid-in Capital
Allocation of net cash proceeds
Net proceeds - Initial Fair Value Allocation	2,108
Per balance sheet upon issuance	2,108
Series A Warrants
Allocation of net cash proceeds
Net proceeds - Initial Fair Value Allocation	4,859
Per balance sheet upon issuance	4,859
Series B Warrants
Allocation of net cash proceeds
Net proceeds - Initial Fair Value Allocation	2,746
Per balance sheet upon issuance	$ 2,746