Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (33,183)	$ (28,592)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	390	474
Amortization of intangible assets	500	499
Stock-based compensation	7,739	9,988
Net premium amortization on investments	(458)	(100)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(658)	(683)
Accounts payable	544	499
Accrued expenses	633	623
Other assets	(2,734)
Other current and non-current liabilities	2,238	(296)
Net cash used in operating activities	(24,989)	(17,588)
Cash flows from investing activities:
Purchases of property and equipment	(645)	(243)
Purchases of investments	(72,981)	(39,799)
Maturities of investments	57,000	35,065
Sales of investments		24,875
Net cash provided by (used in) investing activities	(16,626)	19,898
Cash flows from financing activities:
Proceeds from issuance of common stock under employee stock purchase plan	423	327
Tax payments related to shares withheld for vested restricted stock units	(613)	(115)
Proceeds from exercises of stock options	272	423
Net cash provided by financing activities	82	635
Net increase (decrease) in cash and cash equivalents	(41,533)	2,945
Cash and cash equivalents at beginning of period	51,103	3,386
Cash and cash equivalents at end of period	9,570	6,331
Supplemental disclosure of noncash investing and financing activities:
Change in unrealized gains (losses) on available-for-sale securities	9	48
Equipment purchased in accounts payable and accrued expenses	$ 219	$ 39