Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (21,437)	$ (17,831)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	300	309
Amortization of intangible assets	333	333
Stock-based compensation	5,060	6,595
Net premium amortization on investments	(291)	(51)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,129)	(749)
Accounts payable	305	720
Accrued expenses	282	130
Other asset	(2,726)
Other current and long-term liabilities	2,279	(194)
Net cash used in operating activities	(17,024)	(10,738)
Cash flows from investing activities:
Purchases of property and equipment	(112)	(171)
Purchases of investments	(55,068)	(29,110)
Maturities of investments	32,000	18,315
Sales of investments		24,875
Net cash provided by (used in) investing activities	(23,180)	13,909
Cash flows from financing activities:
Proceeds from issuance of common stock under employee stock purchase plan	222	188
Tax payments related to shares withheld for vested restricted stock units	(613)	(115)
Proceeds from exercises of stock options	272	164
Net cash provided by (used in) financing activities	(119)	237
Net increase (decrease) in cash and cash equivalents	(40,323)	3,408
Cash and cash equivalents at beginning of period	51,103	3,386
Cash and cash equivalents at end of period	10,780	6,794
Supplemental disclosure of noncash investing and financing activities:
Change in unrealized gains (losses) on available-for-sale securities	$ 23	$ 51