Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019
Operating activities
Net loss	$ (3,430)	$ (3,801)	$ (6,079)	$ (7,224)	$ (10,151)	$ (19,981)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation					1,352	3,556
Non-cash discount amortization on term loan					822	247
Change in assets and liabilities:
Prepaid expenses and other assets					562	(520)
Accounts payable					633	(1,674)
Accrued and other liabilities					(43)	574
Net cash used in operating activities					(6,825)	(17,798)
Financing activities
Repayment of principal					(10,259)	(10)
Exercise of stock options						84
Proceeds from sale of common stock						25,150
Offering costs						(2,093)
Net cash (used in) provided by financing activities					(10,259)	23,131
Net (decrease) increase in cash, cash equivalents and restricted cash					(17,084)	5,333
Cash, cash equivalents and restricted cash at beginning of period		18,954		18,473	18,954	18,473
Cash, cash equivalents and restricted cash at end of period	1,870		23,806		1,870	23,806
Supplemental disclosure of cash flow information:
Cash paid for interest					75	358
Supplemental non-cash financing transactions:
Issuance of warrants in connection with term loan						196
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents							$ 1,855
Restricted cash							15
Total cash, cash equivalents and restricted cash	$ 1,870	$ 18,954	$ 23,806	$ 18,473	$ 18,954	$ 18,473	$ 1,870