Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (11,872)	$ (10,078)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	104	162
Amortization of intangible assets	166	167
Stock-based compensation	2,626	2,361
Net premium amortization and discount on available-for-sale securities	(4)	(96)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	460	229
Right-of-use assets	71
Other long-term assets	(68)	(107)
Accounts payable	(749)	(72)
Accrued expenses	(425)	98
Lease liabilities	69
Other current and non-current liabilities		6
Net cash used in operating activities	(9,622)	(7,330)
Cash flows from investing activities:
Purchases of property and equipment	(171)	(45)
Purchases of investments	(3,006)	(44,186)
Maturities of investments	13,500	8,500
Net cash provided by (used) in investing activities	10,323	(35,731)
Cash flows from financing activities:
Proceeds from issuance of common stock under employee stock purchase plan	255	272
Proceeds from exercises of stock options	9	222
Net cash provided by financing activities	264	494
Net increase (decrease) in cash	965	(42,567)
Cash and cash equivalents at beginning of period	6,899	51,103
Cash and cash equivalents at end of period	7,864	8,536
Supplemental disclosure of noncash investing and financing activities:
Change in unrealized gains on available-for-sale securities	$ 13	$ 3