Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (14,418)	$ (11,026)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	351	563
Accretion of short-term investments	(317)	(55)
Stock-based compensation expense	801	914
Stock issued for consulting services	169	308
Change in fair value of derivative liability		(12)
Changes in operating assets and liabilities:
Prepaids and other current assets	(653)	(605)
Other assets	(88)	9
Accounts payable	120	(876)
Accrued expenses	858	(1,058)
Deferred revenue		(3)
Operating lease liabilities	(252)	(193)
Net cash used in operating activities	(13,429)	(12,034)
Cash flows from investing activities:
Purchases of short-term investments	(20,844)	(7,957)
Proceeds received from maturities of short-term investments	37,000	8,850
Purchases of property and equipment	(9)
Net cash provided by investing activities	16,147	893
Cash flows from financing activities:
Proceeds from exercise of stock options	3	21
Net cash provided by financing activities	56	46
Net increase (decrease) in cash, cash equivalents and restricted cash	2,774	(11,095)
Cash, cash equivalents and restricted cash at beginning of period	29,638	31,674
Cash, cash equivalents and restricted cash at end of period	32,412	20,579
Supplemental disclosure of non-cash investing and financing activities:
Deferred offering costs included in accrued expenses		160
Addition of Operating Lease Right-Of-Use Asset [Member]
Supplemental disclosure of non-cash investing and financing activities:
Addition of operating lease, right-of-use asset		15,304
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 53	$ 25