CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash Flows from Operating Activities:
Net income (loss)	$ 8,817	$ (10,974)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Stock-based compensation	750	1,016
Warrant liability revaluation income	(1,101)
Future tranche right liability revaluation income	(20,711)
Issuance of common stock for services rendered	26	23
Accretion of discounts on short-term investments	(18)	(181)
Unrealized gain on available-for-sale securities		2
Depreciation and amortization expense	24	35
Gain on disposal of property and equipment		(8)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,313	(576)
Accounts payable, accrued expenses, and other liabilities	148	(2,694)
Other	4
Net cash used in operating activities	(10,748)	(13,357)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(5,535)	(35,485)
Proceeds from maturity of available-for-sale securities	2,749
Proceeds from the sale of property and equipment		8
Purchases of property and equipment	(7)	(4)
Net cash used in investing activities	(2,793)	(35,481)
Cash Flows from Financing Activities:
Proceeds from common stock financings, net	1,406	1,585
Proceeds from employee stock purchases	25	26
Other		(6)
Net cash provided by financing activities	1,431	1,605
Net decrease in cash and cash equivalents	(12,110)	(47,233)
Cash and cash equivalent, beginning of period	40,019	71,431
Cash and cash equivalents, end of period	27,909	24,198
Supplemental disclosure of cash flow information:
Increase to right-of-use asset upon adoption of ASC 842		261
Increase to lease liability upon adoption of ASC 842		$ 261
Supplemental disclosure of non-cash financing and investing activities:
Capitalized offering costs in accounts payable and accrued expenses	$ 260