Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (36,101)	$ (33,183)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	310	390
Amortization of intangible assets	499	500
Stock-based compensation	7,686	7,739
Net premium amortization and discount on available-for-sale securities	(6)	(458)
Loss on disposal of fixed assets	119
Gain U.S. Treasury securities	(8)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	431	(658)
Right-of-use assets	352
Other long-term assets	129
Accounts payable	(595)	544
Accrued expenses	1,352	633
Lease liabilities	(104)
Other assets	(174)	(2,734)
Other current and non-current liabilities		2,238
Net cash used in operating activities	(26,110)	(24,989)
Cash flows from investing activities:
Purchases of property and equipment	(162)	(645)
Purchases of investments	(25,516)	(72,981)
Maturities of investments	4,510	57,000
Sales of investments	17,000
Net cash used in investing activities	(4,168)	(16,626)
Cash flows from financing activities:
Proceeds from issuance of common stock under employee stock purchase plan	490	423
Proceeds from exercises of warrants	112
Proceeds from exercises of stock options	349	272
Proceeds from the issuance of common stock, net of issuance cost of $565	29,498
Tax payments related to shares withheld for vested restricted stock units		(613)
Net cash provided by financing activities	30,449	82
Net increase (decrease) in cash	171	(41,533)
Cash and cash equivalents at beginning of period	6,899	51,103
Cash and cash equivalents at end of period	7,070	9,570
Supplemental disclosure of noncash investing and financing activities:
Change in unrealized gains on available-for-sale securities	(3)	9
Unpaid property and equipment included in accounts payable and accrued expenses	263	$ 219
Issuance costs for rights offering in accounts payable	$ 64