CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities:
Net loss	$ (56,515)	$ (59,881)	$ (65,984)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,845	5,674	10,720
Warrant liability revaluation expense	598
Future tranche right liability revaluation expense	10,964
Issuance of common stock for services rendered	129	97	150
Accretion of discounts and premiums on investments	(372)		94
Depreciation and amortization expense	120	432	746
(Gain) Loss on disposal or impairment of property and equipment	(10)	477
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,160)	2,717	(1,962)
Accounts payable, accrued expenses, and other liabilities	(1,105)	(866)	1,674
Deferred revenue		(566)	(697)
Other	8
Net cash used in operating activities	(44,498)	(51,916)	(55,259)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(44,502)
Proceeds from maturity of available-for-sale securities	42,100		28,270
Proceeds from the sale of property and equipment	11	290
Purchases of property and equipment	(11)	(75)	(206)
Net cash (used in) provided by investing activities	(2,402)	215	28,064
Cash Flows from Financing Activities:
Proceeds from private placement	10,072
Proceeds from common stock financings, net	5,298		53,763
Proceeds from employee stock purchases	121	243	253
Proceeds from exercise of common stock options and warrants	3	10,166	5,444
Payments on note payable		(209)	(292)
Payments on capital leases	(6)
Payments on capital leases		(8)	(11)
Net cash provided by financing activities	15,488	10,192	59,157
Net (decrease) increase in cash, cash equivalents and restricted cash	(31,412)	(41,509)	31,962
Cash, cash equivalents and restricted cash, beginning of period	71,431	112,940	80,978
Cash, cash equivalents and restricted cash, end of period	40,019	71,431	112,940
Supplemental disclosure of cash flow information:
Cash paid for interest		9	42
Increase to right-of-use asset at adoption of ASC 842 and upon lease reassessment	1,236
Increase to lease liability at adoption of ASC 842 and upon lease reassessment	1,236
Supplemental disclosure of non-cash financing and investing activities:
Non-cash property additions			150
Accrued financing transaction costs	$ 165	$ 101	$ 17