Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (10,078)	$ (8,668)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	162	153
Amortization of intangible assets	167	166
Stock-based compensation	2,361	3,420
Net premium amortization and discount on available-for-sale securities	(96)	(10)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	229	44
Other long-term assets	(107)
Accounts payable	(72)	162
Accrued expenses	98	(188)
Other current and long-term liabilities	6	(97)
Net cash used in operating activities	(7,330)	(5,018)
Cash flows from investing activities:
Purchases of property and equipment	(45)	(34)
Purchases of investments	(44,186)	(19,157)
Maturities of investments	8,500	6,315
Sales of investments		24,875
Net cash provided by (used in) investing activities	(35,731)	11,999
Cash flows from financing activities:
Proceeds from issuance of common stock under employee stock purchase plan	272	188
Proceeds from exercises of stock options	222	112
Net cash provided by financing activities	494	300
Net increase (decrease) in cash	(42,567)	7,281
Cash and cash equivalents at beginning of period	51,103	3,386
Cash and cash equivalents at end of period	8,536	10,667
Supplemental disclosure of noncash investing and financing activities:
Change in unrealized gains (losses) on available-for-sale securities	$ 3	$ 48